It’s a fairly simple moral question. It’s one thing to consent to your own cards being cleaned this way. It’s a another thing entirely to consent on behalf of other people. For example, the number of cleaned cards I want in my collection is 0. So any instance of me buying a cleaned card is an act of deception and someone along the chain of custody of this card has consented for it to be cleaned on my behalf.
Put in other terms, imagine if I thoroughly wiped every card I own on my testicles before grading them. PSA is not going to know because they aren’t sophisticated enough to differentiate finger oils and testicle oils, right? If I only do this to my own cards then it’s strange but everyone along the chain of custody has consented to the process. If I were to sell my teabagged cards, I think most people would agree they would rather not own these particular copies even if they can’t distinguish them. And if you’re a teabagger and it is publicly revealed, can you blame people for not wanting to ever own a card you graded?