Earlier quoted context omitted.
Seems unethical to me, strictly speaking. Sure, everyone's doing it. Doesn't mean cheating someone else out of their fair share is right, though.
"Fair share"? As if each person deserves to be served? Talk about a sense of entitlement. The restaurant will serve whomever it likes. You wouldn't think it unethical for the restaurant to give regulars a friendlier smile, or a specific table they like? If you have a personal relationship with someone, no matter how tenuous, they will give you better service. It's not "cheating." Especially for busy restaurants, ther…
But I'd like to try to reframe this, to focus on what is lost, and who pays the price.
I value transparency and predictability in (most) transactions; I want shopping to be easy, and so I'd rather pay a non-negotiable $100 for something, versus see a pricetag of $200 that I know is negotiable in the range of 30%-70%. (Admittedly, if we're talking about $100,000, I start being interested in the $200,000 plus haggling).
As such, if I learn that a certain business has a non-transparent cost structure, including this tipping-oriented one, I'm disinclined to shop there. So if I witness you bribing the staff, the business owner loses some of my goodwill (even more if you bribe the staff to cut ahead of me in line), which is a Real Loss for them. I'll acknowledge that perhaps the business owner would rather have the business of the bribing type, for whatever reason (perhaps they're bigger spenders).
I guess my conclusion here is that it is unethical for an employee to accept bribes, if and only if they have been given (explicit or implicit) work direction to not do so.
I don't have much to say about whether it's ethical to bribe them, except of course bribing them for unethical gain (e.g. bypassing safety inspections) would be unethical, and maybe it's unethical to incent someone to behave unethically?