That's a good point, yes. Though I have to wonder how vague the orders were - I'd be surprised (but not disbelieving) if Lee's orders explicitly stated Stuart should take all or most of his cavalry off on the ride.
When I last looked into it several years back I found it was Longstreet whose bright idea it was. If I may just copy-paste, because I haven't time to precise:
Stuart's command of 3 brigades was ordered to screen the right flank of the advance.
Robertson's command of 2 brigades was ordered to screen the rear.
Jenkin's brigade was attached to Ewell's Corps and in the advance guard.
Imboden's brigade was sent over to the left flank of the army's advance.
It's a sensible arrangement, cavalry screening each direction, with the strongest bodies in the direction of the expected threat. The problem was his right flank force wandered off. Lee's orders were:
"HEADQUARTERS, June 22, 1863.
Major General J. E. B. STUART,
Commanding Cavalry:
GENERAL: I have just received your note of 7:45 this morning to General Longstreet. I judge the efforts of the enemy yesterday were to arrest our progress and ascertain our whereabouts. Perhaps he is satisfied. Do you know where he is and what he is doing? I fear he will steal a march on us, and get across the Potomac before we are aware. If you find that he is moving northward, and that two brigades can guard the Blue Ridge and take care of your rear, you can move with the other three into Maryland, and take position on General Ewell`s right, place yourself in communication with him, guard his flank, keep him informed of the enemy`s movements, and collect all the supplies you can for the use of the army. One column of General Ewell`s army will probably move toward the Susquehanna by the Emmitsburg route; another by Chambersburg. Accounts from him last night state that there was no enemy west of Frederick. A cavalry force (about 100) guarded the Monocacy Bridge, which was barricaded. You will, of course, take charge of [A. G.] Jenkins` brigade, and give him necessary instructions. All supplies taken in Maryland must be by authorized staff officers for their respective departments-by no one else. They will be paid for, or receipts for the same given to the owners. I will send you a general order on this subject, which I wish you to see is strictly complied with.
I am, very respectfully, your obedient servant,
R. E. LEE,
General. "
Which was passed through Longstreet (to whom Stuart was attached). Longstreet suggested a "ride around", but suggested it was Lee's idea:
"HEADQUARTERS, June 22, 1863-7. 30 p. m.
General R. E. LEE,
Commanding, &c.:
GENERAL: Yours of 4 o`clock this afternoon is received. I have forwarded your letter to General Stuart, with the suggestion that he pass by the enemy`s if he thinks that he may get through. We have nothing of the enemy to-day.
Most respectfully,
JAMES LONGSTREET,
Lieutenant-General, Commanding. "
"
HEADQUARTERS, Millwood, June 22, 1863-7 p. m.
Major General J. E. B. STUART,
Commanding Cavalry:
GENERAL: General Lee has inclosed to me this letter for you, * to be forwarded to you, provided you can be spared from my front, and provided I think that you can move across the Potomac without disclosing our plans. He speaks of your leaving, via Hopewell Gap, and passing by the rear of the enemy. If you can get through by that route, I think that you will be less likely to indicate what our plans are than if you should cross by passing to our rear. I forward the letter of instructions with these suggestions.
Please advise me of the condition of affairs before you leave, and order General Hampton-whom I suppose you will leave here in command-to report to me at Millwood, either by letter or in person, as may be most agreeable to him.
Most respectfully,
JAMES LONGSTREET,
Lieutenant-General.
N. B. -I think that your passage of the Potomac by our rear at the present moment will, in a measure, disclose our plans. You had better not leave us, therefore, unless you can take the proposed route in rear of the enemy. "
and hence afterwards Lee allows it if it won't compromise his primary mission:
"HEADQUARTERS ARMY OF NORTHERN VIRGINIA, June 23, 1863-5 p. m.
Major General J. E. B. STUART,
Commanding Cavalry:
GENERAL: Your notes of 9 and 10. 30 a. m. to-day have just been received. As regards the purchase of tobacco for your men, supposing that Confederate money will not be taken, I am willing for your commissaries or quartermasters to purchase this tobacco and let the men get it from them, but I can have nothing seized by the men.
If General Hooker`s army remains inactive, you can leave two brigades to watch him, and withdraw with the three others, but should he not appear to be moving northward, I think you had better withdraw this side of the mountain to-morrow night, cross at Shepherdstown next day, and move over to Fredericktown.
You will, however, be able to judge whether you can pass around their army, without hinderance, doing them all the damage you can, and cross the river east of the mountains. In either case, after crossing the river, you must move on and feel the right of Ewell`s troops, collecting information, provisions, &c.
Give instructions to the commander of the brigades left behind, to watch the flank and rear of the army, and (in the event of the enemy leaving their front) retire from the mountains west of the Shenandoah, leaving sufficient pickets to guard the passes, and bringing everything clean along the Valley, Closing upon the rear of the army. As regards the movements of the two brigades of the enemy moving toward Warrenton, the commander of the brigades to be left in the mountains must do what he can to counteract them, but I think the sooner you cross into Maryland, after to-morrow, the better.
The movements of Ewell`s corps are as stated in my former letter. Hill`s first division will reach the Potomac to-day, and Longstreet will follow to-morrow.
Be watchful and circumspect in all your movements.
I am, very respectfully and truly, yours,
R. E. LEE,
General. "
Stuart failed to perform his primary mission. Lee allowed Stuart to follow Longstreet's suggestion if it didn't compromise the main effort. It did but he did it anyway. Now you might make the case Lee should not have allowed any latitude, but if Stuart was perceived to be that unreliable then that speaks of something else...