In some sense this was basically inevitable. Robinhood allowed (perhaps even encouraged) a huge mass of clueless investors to flood into the market. As long as the going was good, nobody complained, but as soon as things go south, people have started to learn expensive lessons.
This is not the first time this has happened. Almost every single market stress for the past 20 years has caused grief to retail in some way as they were caught out on something they didn't know about.
One clear example that comes to mind is when the Swiss national bank has removed the currency peg and a whole bunch of FX retail has made the expensive discovery that stop limit orders are not guaranteed to execute...
They're not investing in anything, their money doesn't contribute to any form of progress.
And it was impossible on RH and other brokers to buy more shares, selling (liquidating a long position) was always possible, AFAIK.
More details on margin calls here:
RH is getting sued for market manipulation. This is the inevitable result from that. RH may very well be guilty in all of this, but not for these margin calls.