Not necessarily "nefarious", but I certainly don't trust everyone to behave reasonably or logically. The very act of putting the watermark in will plant that idea in at least some of your customer's heads. And since you didn't answer my question about whether you informed customers of it before they made a purchase, I think it's safe to assume you didn't. Even if all of your customers trust you to act reasonably, you could die tomorrow and all of those rights will get transferred to someone else who may not be as trustworthy.