To some degree I think that it is a question of terminology and cultural habits.
The massage shop has their charge for the massage. The provider may or may not get paid something from that. The bar has an off fee. I do not think that the provider gets paid from the off fee.
The payment to the provider is called a tip, but it is really the amount that the provider charges for his services. There is also this term, minimum tip. But that is also a misnomer as vinapu has explained in the past. The minimum tip is what the provider expects to be paid for his services.
So the actual tip is whatever you decide to give above and beyond the amount that the provider charges for his services. You can give whatever you want, but in most contexts (restaurant, taxi, haircut, etc) a 20% tip would be a large tip.
If you didn’t really care for the provider’s service, you pay the agreed upon provider service charge and move on. No tip necessary.
Americans tip excessively. That is the cultural part of this.
The first world vs third world dynamic also plays into this, particularly if you have a fondness for the provider. Some will give more money to the waiter, taxi driver, massage guy or escort, because they think that the provider could use the money and the client has the money to give.