The model doesn't fall apart legally because it can be trivially circumvented. Any more than the model of renting bikes falls apart legally if people just don't bring the bikes back, or the model of selling software licenses falls apart legally because people can easily copy it. You can base the livelihoods of people on these things, provided the amount of circumvention remains low.
I see what you're saying, but I wasn't intending to stretch my thought that far. I was just assuming that changing the referral header couldn't possibly be illegal.
