Likely this will be implemented at the ISP level, I expect 99.99 of the sites would not care otherwise.
It's basically the same problem as preventing online piracy.
"good enough" here isn't good enough.
I'm going to draw parallels with online piracy again, where countless DRM and legal (both through legislation and litigation) efforts are made to curb piracy yet rarely make a dent on the numbers of people who access copyrighted content illegally via one means or another.