The problem with accusing people of cheating without evidence (as if it needs pointing out) is that it allows you to claim anything. If I claim in public that Biden murders babies in a secret torture chamber at his house, and the police go to his house and find no torture chamber, I should then be punished for making a libellous false allegation. I shouldn't be allowed to defend myself by saying 'hurr durr useless idiot cops, he was too clever to let you find it.'