not disagreeing with you re etiquette of some trainers, but why does a trainer who is paying rent to a gym need to go over to folks and correct their form? yes, there are trainers who do this in an attempt to generate business, but it's not up to a trainer who is not on the payroll of the gym to correct every members form. earning fees for their services is how trainers make a living. many gyms have cert 3 or 4 qualified staff employed by the gym itself to be a presence on the floor and help members with one-off questions and correcting form immediately if need be.
I've generally found even the employed trainers say nothing to anyone who isn't giving them $$$. I'm not sure how it all works, i assume it is a base rate and commission based or something?
its like everything i guess if the advice is good you want to hear it. Chain gyms rarely have trainers who give good advice (unless its about how to pose in the mirror after doing your bicep curls)