You're right about them not allowing different tires being run and et cetera. Not trying to speak for Garrell, but I think the main point is the s&p guys are running on the cheap- be it by using a set of tires longer or buying less of them, period (after all they don't run the race, all they need to do is practice just enough to qualify).
I bet they'd do it if they could, though.
But too much chance the tires would be treated w/traction compound, or competitive tire makers would get a hold of them while they're still 'fresh' (they can get tires, but they're gonna be well used or flat/worn plumb out) or like you said they'd be used for clandestine testing.
What I'd like to see is a complete all-in-one-place run-down of the pay scale including all the different programs, contingencies and incentives, etc. that make up a NASCAR race payout. It used to be simple- some big name guys got "appearance money" and whatever share of the purse their finishing position earned them. Now you'd prolly need to be a CPA to figure it all out, but IIRC there are guys who can finish dead last and make more than a guy who finishes top-ten.