To me there are two sides to this coin. From the escorts side, the escort was contracted for a service. From the john's side, the john is having sex. Those are different things. Same act, two separate objectives.
If the john doesn't pay the escort, then maybe in his fantasy world he really was having sex, and not paying for a service. Who knows? (Who cares?)
However, if the escort doesn't get paid, the john has voided the contract for a service. And the 'sex' is no longer an agreed upon service, it's just rape.
The escort didn't agree to have sex, he agreed to be paid for a service. No free samples were on offer.