What @eoinfinnegan says is spot on accurate. In everyday life, you can be the most honest person in the world and it doesn’t change things. As you go through your day, unless you live alone in a cabin deep in a forest, you are constantly paying for the community around you.
As far as Fiverr goes, there is no real way to tell who would break the rules and who wouldn’t so the 20% has to apply to everyone. Very honestly, I don’t mind it one bit.
I tried selling on several sites before Fiverr and didn’t make any money. I spent some money on “credits” or advertising, etc. Fiverr doesn’t cost me even a dollar unless I make four dollars for myself. When I work harder by bidding on Buyer Requests or promoting myself, I usually make more money. If I worked at a “regular” job they would take out taxes, insurance and all kinds of things. Tip or service, Fiverr seems very fair to me.