You already gave the very definition of every internet gambling service in existence.
A good gambling service just skims enough money off its addicts using different smoke screens/deceptons and complex algorithms to preserve the illusion of fair play and make them return even after losses.
That's how these services make money, due to math (stochastic) that's the only way without running openly visible fees.

If you believe there's a mathematically non-tilted, non-rigged BTC gambling service out there - sorry to break this to you, but there isn't. To even consider otherwise would be downright naive.
The entire gambling industry out there serves only one purpose : find fools with that addiction and skin them (some more, some less), that's what it's all about.

Short of the kinky / SM industry, it's the biggest industrialized abuse of customers in the world, so to speak. Definitely the oldest of them.

The millenia old saying doesn't exist for no reason : gamblers are fools (because they volunteer to be taken advantage of)
Arguably, one could call it a "fee" for their entertainment, but the only incentive of gambling service operators has only been money - since the dawn of time...