It's funny (kind of cute, honestly) that these site operators pretended that the outbound (booting) side of the service was the only legal risk, and that they could address this with click-through terms. Clearly, compromising third-party devices and services, or misusing services for amplification, is just as legally fraught as the attack itself.
That being said, I wonder if these services are actually the limiting factor here. There is probably some zero-sum game here, with a fixed quantity of exploitable booter hosts available and all the providers vying for control of these. Shutting down a set of providers would then just make others more powerful.