The formal term for tip is "gratuity" - which comes from the old French word gratuité, by way of the Latin gratuitus, meaning "voluntary".
And that's the very point of it: a tip is a voluntary payment, made in exchange for service.
No service, no tip. The better the service, the larger the gratuity, as determined voluntarily by the customer.
Where such payments are either banned or mandated, the incentive for a service provider to provide superior service is diminshed.
Personally, I leave 15% as a standard amount for an acceptable level of service (somewhere in the middle of the quality bell curve). I will leave 20% for superior service, and 10% or less for an inferior customer experience.
On occasion, I have left more than 20%, and as little as 1 cent. As leaving nothing might be misinterpreted as an oversight, I have found that the remittance of a single penny in the event of truly abysmal service makes the point with unmistakable clarity.