They are legitimate but as Richard said they are often autographs on a plastic strip that is affixed to a card. Some are written on the card themselves. I do not particularly like the strip signed ones but in looking at the picture of a card you can usually see the outline of the strip. I think the theory behind having strips signed instead of cards is they can change card images.
IV only got one pop century in my collection. I'd prefer on card signed instead of the signatures on the clear stickers but they do put out some cool cards