Fiverr takes 20 percent of all seller income - regardless of its source.
While I think your tone is a bit over the top, I don’t disagree with you about the ill-feeling Fiverr taking commission on tips generates among some sellers.
However, in the real world outside of Fiverr, ask yourself how many freelancers receive tips? Very few in my experience. It rarely happens. A freelancer and a client agree a fixed fee for a job - and that’s it.
This whole business of tipping is actually very odd in the freelance world outside of Fiverr - and I can only therefore deduce it’s simply another way for Fiverr to make more money.
In other words, regardless of the fact that Fiverr take 20 percent, in my opinion you should be grateful that you’re receiving additional income in the first place!