I don't understand why would they do that, though.
Another issue is that it carries connotation. When you say "This model was photoshopped to look thinner/prettier/etc." you create a negative connotation with the brand. Adobe fears people will hear "photoshop" and immediately think "falsifying images, creating false reality, hiding stuff" etc.
It also causes the brand to be identified with a single product, which in Adobe's case is not such a huge deal, as Photoshop is the product, but in other brands, like Apple, or Google (again), means a whole slew of products across different fields, this is also why the Apple sign was removed from the Macintosh' keyboard.
Actually, it means exactly that-- as you can tell by the number of companies selling "aspirin" or "cellophane" or "linoleum".
Trademark is precisely the protection that prevents others from selling a product with your name (or one so similar as to be easily confused.)