I think Apple is probably going along the lines of brand confusion and similarity of product names. Everybody expected Apple's wearable device to be called the 'iWatch' and 'Apple Watch' was somewhat of a surprise. This doesn't stop some people referring to it as the 'iWatch' in slang. If a company comes out with a product called the 'iWatch', this may well lead to confusion about the product's relationship with Apple. If they snatch up the rights for the name, they avoid any of this.
I'm purely speculating here. I'm no expert.