Because it's not effective.
Anyone with the time, patience, skill, and correct editing software can either; a) Remove the watermark and everything with it, leaving a perfectly viable (and high quality) PDF or b) _change_ the name...
ANY encryption method can be broken with time... So even if they encode xyx information in the watermark, someone could (and probably would) crack it eventually.
Imagine this scenario... you're sitting home gaming with some buds and get a knock at the door...
"Mr so-and-so, you're under arrest for copyright infringement and distribution of pirated material across state lines and country boundries"
Yea... I'm totally opposed to what DT does now, personally... as a publisher I understand why publishers like it.... but being in software I understand that piracy is _going_ to happen...
Sadly there is no all in one answer that someone, somewhere will much up because they're bored or like the challenge. *sigh*