Well, almost half of their subscribers are paid (and they are the real target of the service) and are not going to leave because of this change in the ToS.
The ad supported option only exists to advertise the paid service as I believe they lose money on the free subscribers even with the few ads they play.
So, if you are blocking their ads, they really don't want you anyway, something about them having to pay for each song they stream, and there is no chance you will start a paid subscription.