I suspect that the "service" distinction is no less arbitrary than tipping in general. It might not seem that an occupation has a service component until it vanishes. How can counter service go wrong? Famous last words. If tipping sets up a relationship between quality and pay, it could affect any job.

Who gets tipped is a matter of social custom, though each person is free to come up with their own criteria.

Disclosure: I earn tip income in my side job as a musician.