I agree that this assurance should be a part of the package that is being paid for.In any other contexts that I can think of right now, I'm of the opinion that biology being what it is, the time for a guy to make a choice regarding parental responsibilities is BEFORE he puts his dick in a pussy.
However, in this context, I believe that "no-strings-attached" is part of the package that guys pay for when hiring an escort. In my perspective, the premium that clients pay me for comes with an understanding that our responsibilities to each other start and end within the boundaries of paid-for-time. Clients pay for a fantasy, which in my opinion means a girlfriend who is always happy to see them, always perfectly groomed and coiffed, never bitches about her day, always wants to fuck and suck cock, and importantly, doesn't accidentally get pregnant and expect that the guy take responsibility for the child.
Getting knocked up and going after a customer for child support is a ridiculous business practice.