In theory you're right, but, the current state of the business wouldn't allow you to do this. I live in Manhattan, certainly one of the best places to be a trainer, from a potential client standpoint. However, there are basically no more hardcore gyms, only fitness type chains that would never allow trainers to operate freelance. If you want to be a trainer there, you have to play by their rules. In other words you have to let them ass rape you.
You're right...it can be hard to find the "right" gym.
IMO, you don't want a strictly hardcore gym to work out of...there is no money in that...but they're are still gyms out there that welcome both types of people...every day people and the hardcore guy...that's the gym you want to find.
The gym I work in is a large gym that has everyone from every walk of life in it. It is a small chain, privately owned...there are only 6 of them, so there is not some huge coprate gang rape crowed to screw you in the personal training thing.
Another option and this is getting to be the better option for a lot of trainer...working for a private training studio. Most of the chain ones will still rape you but there are tons of privately owned ones that will pay very well. But here lies the problem for most trainers...most have the idea of getting/wanting to train the next Mr. O and having their clients go through hard core training that they can put up on YouTube. The problem is this is just not reality. You yourself could be an awesome hardcore BB and you may even get to train some up and comers, but your biggest job is training and helping normal people lose weight, helping middle aged house wives get back into shape, etc. This takes a total differant mentality that a lot of trainers just are not willing to accept, and that's why they either fall flat on their face or end up hating their job which in turn makes them act like retards out on the gym floor...in the end, they still fall flat on their face.