Do you mean that wireless providers in Europe are explicitly required to unlock your phone (if you ask, for some fee)? Or that they're explicitly required to provide software updates for unlocked phones? Otherwise, I don't see how the situation is any different in the US than it is in Europe.
In the US, you're perfectly free to unlock your phone. In fact, there's a
specific exemption to the DMCA [PDF] allowing cell phone handset unlocking. You certainly can unlock GSM phones here, or buy unlocked phones (I've done both).
So, are wireless providers required to explicitly offer an unlocking service, or just to not arrest you if you do unlock your phone? The latter is essentially the case in the US.