Every time I've stumbled upon phenomenon of real-life brands, products, works etc. in a fictional work, one thing always bugged me.

Straight to the point - what's the big deal with twisting the names of real life products in fiction? For example in certain webcomic (which I'm not gonna mention here :P) characters went to the cinema to watch... "Dragon liver", which was an obvious parody of a movie "Dragon heart".

I thought the reason for such modification of the name might be some sort of legality issues, but then, in that same webcomic there's a character who is a fan of Star Trek, and the author makes no effort whatsoever to distinguish it from the real life Star Trek show. It even goes as far as the character openly commenting upon the show, possibly displaying the author's own opinion about ST.

The question is - what is the point of twisting names of real life brands and product names? Can you be sued for making a trademarked product part of your story or something? Or can you make your story's characters drink Coca Cola without any fear of legality problems? I honestly don't understand this...