Attendants are employees of the franchise owner and receive a salary plus tips. Some despachadores (attendants) receive as little as 685 pesos a week but do receive benefits provided by the law.
I can tell you from personal experience that IMSS sends inspectors to businesses on a regular basis to check. If they are working for a company big or small and do not receive the benefits (SS payments, SS medical, Infonavit, etc.) the owner will be fined big time.
We were fined more than 100,000 pesos in 1996 for thinking our workers had benefits paid for by the contractor and turns out they didn't. We were able to reduce the fine by visiting the IMSS office and making monthly payments for a year. Shame on us for being so naive. Even if you pay off the inspector to reduce the fines you still have to register everyone. They take names :)
Tips are a big part of their income though.