bcs WE WANT the game in best shape possible? without stres (or other) the game would be in much worse state, there are some things you cant test with just couple people, you need thousands, and blizz cant possibly have so many paid testers, thats why beta/ptr exists in the first place...
without our "free" help, only thing that would happen would be game on release having FAR MORE bugs and issues, which blizz would repair then, so honestly, we would be much more affected by not giving the free help than blizz themselves...
so we are not helping blizzard, but OURSELVES and other players