> it would nonetheless be tempting to consider using it were I a landlord.
Of course it would. I still don't like this practice.
> is there any protected status in the US for "people that make use of the court system" that would bar such discrimination?
There are certain laws that protect people who use the court system in some ways, but there is not (to my knowledge) a general protection. But whistleblowers, litigants in harassment claims, etc. have anti-retaliatory protections.