I am confused why did they operate then?
We are in a pandemic and if people realized they have the virus they will find a way to blame it to others even if they are at fault and obviously, they will blame the casinos or maybe the bars. Despite the level of the casinos, small or not, there will be no exceptions. They should've think this through.
Maybe that is the reason why some casinos, despite the permission for them to operate did not open.
Is it allowed in the US to sue entities for the spreading of "common" diseases such as the flu or the COVID ?
Covid, yes.
As far as I know these casinos or bars shoyld be able to follow the protocols so that people would be safe from the pandemic. If it is proven that they are not following these protocols, yes, they can sue the casinos. As far as I know workers are even suing the casinos where they are working with the same resaon.