I recently posted about a dozen photos from a wedding on FB with a watermark across the bottom - the bridesmaid copied them cropped the watermark off and reposted them in her album - when i pointed out (nicely) that she was breaching copyright and a contract i had with the bride and groom - she reposted the originals. - In the main i think it was ignorance - more than an effort to make them appear to be hers - after all she was un some of them ...

Unfortunately many people just don't realise (or care) that its theft - and a little watermark at the bottom really doesn't protect you alot - people crop and don't really care about the effect that that has on the image.