I don't disagree (nor totally agree) with your opinion about the matter. But they don't take a 20% of all their freelancers earnings, they take 20% on the first $500 of your per-client earnings, then %10 of the next $9500, then %5 of any more work with your client. Also, you are free to work with any client outside the platform 2 years after you start work with them.