It conditions the dog to be ready for anything during a stay and to really hold on tight. For most dogs the first time you do it the dog will kind of stumble a little bit and won't really push back because it is unexpected. The dog quickly learns to hold tight and tense his/her leg muscles when approached by someone who is going to touch him/her during a stand stay. Personally, this is exactly what I want the dog to do during a stand for exam. You are capturing (well not really, becasue you are inducing the reaction, but you know what I mean) and shaping the opposition reflex, and yes rewarding it, because during a stay this is a desirable reaction.
Of course you don't reward pulling on the leash or harness and the comparison is a little far fetched in my opinion. If your dog is brilliant enough to think "I push against a little pressure from mom's hand on my shoulder when I'm trying not to break a stay and get praised for it, so therefore I will get praise for pulling against pressure on a leash during a walk" than you have the amazing human-minded beast!
I am assuming that you are not, however, implying that this exercise will make your dog pull on a leash, but that it is futile to reward an involuntary reflex? In which case, I understand the reasoning, but I think that with repition the involuntary response can become voluntary. I'm babbling now, I will squelch my urge to give examples.