In the simplest, likely potentially inaccurate explanation... Robinhood doesn't actually make the trades. They pay someone else to make the trades, add a small amount to the price, and that's how they get to 0$ trading price to the end user. The upstream party is refusing to do things, so Robinhood has no choice but to follow the rules that party set.

That's like, way over simplified but gets to the gist

[Edit cause spelling are hard]