The thing is that they CAN'T force you to enroll.
You go and buy from people not in the program spend your bitcoin on dice or porn or drugs and they can't do anything.
And nobody said that you can't have a trusted address and an untrusted one at the same time.
Okay, I get it, you mean a black market will keep existing. That's correct. But there will be much more monitoring, and that makes it much more difficult. For example, suppose your salary is payed in BTC. They know exactly how much you have, because your salary is whitelisted. Now you want to buy something they don't approve or donate money to wikileaks, whatever. You know you can't use your whitelisted addresses because they can link them to you. You'd need to exchange your coins against "free coins". But that would leave a trace... you see the problem? By controlling your money, they control you.