I know very little about cat behaviour but according to Bruce Fogle, 'suckling from mother provides a kitten not only with nourishment for six weeks but also with social and behgavoural development for another six weeks. Kittens removed from their mothers before they complete that second six weeks of comfort suckling are more probe to wool sucking. He goes on to say that it makes the sucker feel good and releases dopamine. Were they removed early? Bruce says nothing about face sucking, so unless your face is spectacularly furry there's something else about you they like.