Legally, the employer is supposed to ensure that the wages plus tips equal out to the $7.25 per hour. Legally, they are paying minimum wage. We need the laws changed to fix this issue, but the government doesn't seem primed to make these sorts of changes.
As far as why some jobs attract a tip and others don't, I think it has to do with a combination of custom and what sort of work one does: On top of this, some service sector employers prohibit employees from taking gifts or tips from customers (retail, for example, generally prohibits this and you can get fired for accepting). This obviously has an impact on the norms.
I tip my hair cutter, but not my car mechanic, mostly due to C, I guess.
Interesting you should mention. I've seen evidence where this was tested in a local consumer right program. Someone would disconnect their spark plug, and then act as if they're an innocent driver who's car is broken. Or a piece of rubber of < 5 EUR was removed. Some car technicians were honest (mostly those who were part of trade organisation BOVAG), don't get me wrong. But some also deceived/lied about the problem, and made the customer pay insane prices to fix a simple problem. They also tested if technicians would fix an additional, obvious problem. One even fixed the problem, but made something else worse. In short, I'm not going to assume anymore that car technicians are honestly going to fix all the problems my car has.