It strikes me that the extra fun(if any were needed) come in that watermark 'drm' actively encourages would-be pirates, release groups, and warez kiddies, to go hunting for hapless random users(either to obtain already-purchased copies with some sucker's watermark, or to get account credentials to purchase whatever it is they are looking to release into the wild).
Even if we assume(almost certainly too generously) that the watermarks are indelible, even for an attacker in possession of multiple copies, an attacker doesn't need to strip the watermark they just need to make sure that the watermark isn't linked to them. No need to do some sort of fancy cryptoanalytic attack, just phish firstname.lastname@example.org; buy whatever it is, and send it to TPB.
Any 'protection' scheme that encourages malefactors to prey on paying customers seems like a... potential problem.
(And, of course, though this probably isn't seen as a bad thing by the people who would use something like this, exercising your right of first sale similarly exposes you! Sure, the system doesn't stop you from selling it; but you wouldn't want to get sued when the person you sold it to, or the person they sold it to, hoists the Jolly Roger, now would you?)