I think it's more of a habit than anything else. Sand blasters transition over to soda blasting and carry traits along with it. Having said that it's really a very minor thing to have only that area left for the panel shop to clean up, and sure beats trying to repair a warp or ripple in that area. Better to be safe than sorry.

when we get cars blasted we tend to get the apprentice to strip the bonnet and boot by hand.