All Robinhood accounts default to Reg T margin accounts.
The entire UX is built around this idea that the margin is transparent to end users. How many Robinhood users are aware that when they open an account, transfer money, and buy stocks all in the same day they're actually buying on margin? The cash isn't in their account until the bank transfer settles.
Likewise, when Robinhood users hit sell and then use the funds to re-buy another stock or back into the same stock, they're buying on margin. Technically the first sell doesn't clear for several days, so the subsequent purchase is made on margin.
This frenzy surely pushed the limits of Robinhood's available credit lines to support all of this margin activity.
Meanwhile, clearinghouses everywhere are increasing collateral requirements for stocks like GME. If Robinhood was struggling to secure additional credit while also facing increased fees for trading stocks like GME, they may have had no choice but to either pause the entire platform (to avoid running afoul of SEC-mandated margin requirements) or pausing meme stocks while they figured out another solution.
> Why in the world would you expect RH to act in the interest of their users
If the whole platform blows up because they go insolvent, that's even worse for the users.