Well, you may be pretty understanding of the level at which software is released at its different evolving stages, however, some (most?) people aren't.
If it so happens that the watermark can be removed and then some fanboi/fandroid site publishes videos of a pre-alpha Sailfish, affecting Jolla's image, then we won't get any more pre-alpha stuff to test.

Remember: because some people are naughty, we can't have good things (like bringing your own bottle of water to the airport). So, I hope it isn't removable :-)