If they didn't do that, people could just use casinos as a mixer and there's nothing in it for them.
If people wanted to mix with casinos (for whatever reason) instead of a mixer, they could probably brunt the 1% fee in the form of house edge which I presume would be far less than the amount they are paying an alternative mixer.
Does it stop money laundering? Kind of... but does it also create a massive benefit for the casino's end, ensuring that they get at least a small percentage of all users' deposits? Yes.
A few BTC-based casinos did not have a rollover requirement... I'm not sure if there is a sportsbook out there.