Those same arguments apply to several other professions where it is not customary to tip. The reality is that tips are expected only for the lower social/economic classes, where it is acceptable that the price for labor is set based on the whims of the customer instead of being pre-agreed (which, personally, I find degrading).
Right-leaning people like it because it creates a power dynamic. Left-leaning people like it because they delude themselves that they're helping the working class. In both cases, it boils down to paternalism, and all other reasons are rationalization.
