The initial and watermark is in some attempt to prevent people from stealing your work to pass off as their own. They still can crop it off unless you plaster it across your whole picture, which would make it really ugly and pointless.
My way of approaching the matter is to ensure that not too large a size is posted. Even if they use it for monetary gain, the scope is limited.
I empathised how he felt when a giant company like Avis pinched his pic without giving a hood. Avis may not aware of it if its done by their advert agent. Because of rampant cyberspace theft, I normally post to a max of 650 resolution on the long side for pic which I reckon is of good value. With powerful photoshop, watermark is quite redundant unless you spread it across the pic which will impair any viewing pleasure.
And I'd like to add a personal note to everyone reading this.
If you want to use a watermark on your photos, please at least design/use a decent looking one. Don't let it destroy the image with a lousy design/font/placement.