They're mad that cloudflare cut them without real warning. And they should be! Anyone can get on a big company's bad side, and if there aren't extremely important messages being withheld by the author this makes it scary for anyone to use cloudflare.
If a custom IP is going to be mandatory, they need to say that and give a deadline, at the very least.
The IP-reputation damage is immediate. Cloudflare is choosing to pass the hard landing directly onto their customer instead of forcing their other customers to share the damage.
As a CF customer, I am happy that Cf is preventing another business from damaging mine.
The ToS doesn't say anything against gambling sites. Even if there was IP reputation damage, it's not appropriate to cut them off so immediately. Especially when they're a long-term paying customer.
If a custom IP is going to be mandatory, they need to say that and give a deadline, at the very least.