A bar is purely for entertainment though, and that is what everyone expects. A stock brokerage is a service people count on to be able to execute their trades,

A bar is purely for entertainment though, and that is what everyone expects. A stock brokerage is a service people count on to be able to execute their trades, some people even make a living from it. So it's more like if the electric company just decided to shut off power to your factory one day, or the bank just decided not to open one day that they were supposed to be open and you were unable to withdraw money that you needed. These are things you might reasonably try to sue over if they caused you tangible damages. And even more so if the other party might have a vested interest in stopping you. Of course you might not always win. In the case of Robinhood, since people could still close their existing positions there might not be a very strong case to be able to prove damages. But to me it's still not a crazy lawsuit to attempt based on how many people were expecting to be able to use RH to buy this stock today, and that's what the legal system is there for.