I don’t have a data feed available to verify but it’s at least possible there is no one on the other side of the order book for these symbols so Robinhood can’t execute any more buy orders.
Judging this as a policy decision from a 1 sentence tweet and some screen shots seems premature.
One explanation is that this is a systematic behavior, part of their software is seeing market conditions it can’t execute into and hitting this case. If it’s not sustained or is happening only during particular microbook states the best thing to do is let the software behave as designed and let it stabilize.
If it were a case of their software functioning as intended, it would be malpractice to not come out and say that. Right now it looks like they're deliberately manipulating the market to the detriment of their customers. That makes for a humongous blow to trust in their platform and can lose them many, many customers.
Judging this as a policy decision from a 1 sentence tweet and some screen shots seems premature.