Self-serve ordering terminals already often ask for tips. Presumably to be legal they're being paid to the kitchen staff, but I think sticking to "tips are for workers who have to pretend to like me" is a pretty firm boundary to stick to.
(Also, arbitrarily reclassifying things as tips is hard, because legally 100% of tip revenue has to go to workers, not management, and certainly not the company's investors or coffers).
Tax-free tips paid to robots go to the hardworking AI engineers -> AI engineers voluntarily donate part of their tips to a 501(c)(3) that helps support struggling venture capitalists.
Something like that. We'll work it out the details once the right PAC donations are in place.
Now that tips are tax free, it's only a matter of time before some clever SV accountants figure out how make everything a tip.