The interviewer could have honestly made a mistake thinking the candidate cheated when the candidate did not. Or the candidate thought something was allowed but the interviewer did not communicate that it was not.

At any rate, there's a chance the accusation was wrong, and if you make wrong accusations and the other person disputes it, it could lead to unnecessary legal proceedings. (libel, etc.)

PS: not saying the company policy by the GP is necessarily correct, just saying what lawyers and risk adverse bureaucrats might be thinking when they drafted it.