Earlier quoted context omitted.
No, they don't. You're trying a false equivalency between a bribe and gratuity. A bribe happens before the service is provided and the service is contingent upon the bribe. Gratuity is post factum, technically based on the quality of the service provided.
"Tipping" absolutely is a kind of bribe. Let's take police officers in México, for México, for example. One needs to pay bribes to get anything done. On the other hand, they have almost zero job protection, have absolutely crappy salaries, most of the time have to buy their own uniforms, their own gas for the police cars, and so forth. And on top of that, they have to contribute to "upwards bribes", so they can keep…
No, it is not. Legally, per the SCOTUS, it is definitely not https://en.wikipedia.org/wiki/Snyder_v._United_States