Bars do this all the time in areas where there are problems - they scan your ID and then share the information with other bars, particularly if you get booted.
In the United States, if you make a good faith claim about someone, it's pretty hard to win a libel suit, and, I suspect, it would be even harder to claim that grading an individual rises to the level of slander/libel, particularly if you provide a mechanism to challenge the claims.
If this were not the cases, then credit rating agencies would have a difficult time existing.
While your general point still stands, I don't think the existence of credit rating agencies proves much. Even if they would otherwise have been liable, there fundamental utility would, I'm sure, have led to specific legal exemptions.
Put up that sign and then refuse service to blacks, and you're not fine.
Put up that sign, implement the described policy, receive disproportionate numbers of complaints about blacks (because landlords are racist?), enforce the described policy equally irrespective of race, and you are walking a fine line. You could be sued for that and might not be fine.
In the United States, if you make a good faith claim about someone, it's pretty hard to win a libel suit, and, I suspect, it would be even harder to claim that grading an individual rises to the level of slander/libel, particularly if you provide a mechanism to challenge the claims.
If this were not the cases, then credit rating agencies would have a difficult time existing.