The same reason every other artists puts that thing on his/her pictures. I think some might know that it's no use to put it on it. That's the reason why MysticAlpha and JJ don't use it. However I am pretty sure that the artists are fully aware that this thing ruins it for the public when it is added to their piece
Yeah, it's a pity. But some artists just don't know how to help themselves. And if you thought this is bad here, I saw one with 5 watermarks in total, the deviantart and each twice of the artist name and the link to his page, right in the middle of the otherwise beautiful piece.
Caught up a little on my notices, so I can comment again... and everyone is happy... or at least they pretend, to get a good comment I like the general concept and the background... I see you made Luna in a more "real" style then that in MLP. Not bad, her mane looks a tad messy though Overall - a nicely done work.