Not a scam per se, as you are getting what you paid for (i.e., an independent party appraising the grade from the grading company).
But I find them cringe and don’t believe that they should add value to a card. Buyers should purchase cards that align with their own expectations for each grade. There is too much variance within each grade to blindly trust the numeric value, but adding another person’s opinion doesn’t magically solve it either.
Not that long ago we had a seller advertising a CRC “reconfirmed” 10 to show prospective buyers that PSA still believes that their Base 2 Chorizo is in 10 condition. I could imagine this practice taking off with high-end vintage PSA 10 cards.