Businesses where tipping is customary often factor the expected tip into the pay structure of the employee. Those also tend to be lines of business where the person would go once in a while rather than two or three times a week.
Also: just because a business is in the service industry does not automatically imply that tipping is part of it. You would not tip an accountant or a lawyer, for example.
I agree that a ‘regular tip’ after sessions is not expected by most trainers. Since our client is already paying a ‘fee for our direct service’ there really shouldn’t be any expectation of an additional tip for that same service in my opinion. This is a little different from some other service situations such as a food server since we as the diner are not automatically paying them for their service, but instead for the food, and so the tip IS their pay for service. But, as Karin points out, I view our service more in line with that of other professionals such as lawyers, accountants, and doctors.
I hope this helps.