Google Voice is a mobile phone service provided by the maker of one of the biggest competitors to the iPhone OS. What if Google Voice were instead Microsoft Voice? And what if Windows Mobile were as modern and competitive as Android? Would you be as surprised then that Apple is discouraging iPhone owners from using the service? Just saying.
Except that Google Voice isn’t competing with the platform but building on top of it. Following Gruber’s logic would imply that Apple should restrict their mail client from working with Gmail, Exchange, &c because they compete with their MobileMe service. Except that Apple doesn’t even have a product competing with Google Voice. Actually it’s probably more like saying Apple should try to stop Microsoft from distributing Office for Mac because Microsoft also makes operating systems and therefore competes with Apple. Or something. Just saying.
Bonus link: if you don’t know what Google Voice is (as i did until very recently) and why you’d want it (or not), lifehacker has a nice overview. I’m not holding my breath for this launching in Austria any time soon, though.