Ok sure, but I still don't really see what's so bad about being margin called. Like even given that unlikely occurrence where RH might margin call you, what you get out of RH doing this still seems all around great for the user.
Margin calls typically end poorly for the user because they are highly correlated with the asset being in a negative position.
If you buy a $10 stock on margin, and it raises to $15, you owe Robinhood $10, and that is secured by a $15 stock. This is extremely unlikely to get margin called. Even if it does, you're up.
If you buy a $10 stock on margin, and it drops to $5 (even if just temporarily), you owe Robinhood $10 secured by a $5 stock, which puts them in a riskier position, and may margin-call to cut their losses. You get the $5 from the sale, but still owe Robinhood the initial $10.
I'm sure you can see how this can compound to amplify drops if a large number of people have bought the same thing, all on margin, and it starts dropping for whatever reason.
Edit: Follow-on
This is particularly "bad" if your goal was to "own the stock" (at pretty much any price) just so that someone else can't buy it.
