Because "consumer choice" isn't a practical and full response to corporate abuse of power. Regulation is complete - if it's a well written regulation it works.
Amazon has a dominant position in the marketplace and it is leveraging it in a blatantly anti-competitive manner. As a country we have consistently decided that smaller and decentralized is better - there is no compelling reason to allow Amazon to keep borging small businesses.
"Nobody is forcing you" misses the point.