At a spa, the bodyworkers are usually hourly/daily wage-eating employees, with taxes withheld. Most legit ops require proof of education and certification in the field, which is at the prior expense of the worker. They are on call with clients assigned, all overhead and expendables covered, and tips for them can go a long way to get them through hours cut in slow periods. If they have repeat requests by name, more hours will come their way. Some may also do private legit bodywork on the side, but some may not have a home setup for it, and others may even be concurrently attending school for PT or other further medical training, without the time to earn more than the spa pays.
If you're not inclined to be a service tipper, then don't. It's not really a problem until your next visit...