I think it should be illegal to ask for a variable tip before a service is substantially completed. Go ahead and charge a fixed gratuity, but prepaying tips is silly.
It definitely shouldn't be unlawful for the service provider to ask prior to completing the service but I really wish more people would see it as distasteful, as seemingly most forms of drip pricing are considered. Obviously the tip is for exceptionally good service; if it's baked into the sale price then it's just another form of drip pricing.
I wouldn't burden the service provider, I would burden the payment processor and card network. If there’s a license involved, even better. Some merchant codes can’t have tips, merchant onboarding can’t have tip upselling by the sales team
Regulate behavior by regulating the intermediary
Think about anything this way and you can push the same changes through any form of government
