She mentions that the contract also entitled her to a "production bonus", which sounds like some kind of lump-sum payment if her book is made into a movie.

Is it because people are that inexperienced, that some rookies will still fall for net profit thing? Heck, I'm not in film, and even I know not to ask for net. :P

Sometimes inexperience. Sometimes you just don't have the leverage.

In this instance, she probably got paid some fixed amount up front and wanted her name in the credits.

From reading the article, it really sounds like the only thing that's contentious is the fact that they didn't give her credit. She probably assumed that the whole percentage of net was actually as useless as expected.

Since any contract can be full of tricks, it just means you need an army of lawyers even for the simplest things. It then gets really hard to actually do anything. And if you're poor, you're bound to get exploited by some obscure loophole.

For example, one of Peter Jackson's main grievances when he sued New Line was that he had been promised a cut of the revenue from merchandising right sales. New Line sold the merchandising rights to its own sibling and subsidiary companies using a closed bidding process at well below market rates.

Also, I can't imagine any of my friends still working with them after I've been screwed over.

Isn't this a relatively small world? Why work with people like that?

It works because they can. They do it because they've figured that out. It's that simple.