You can not sign away your rights. If you sign a document that said your company can just kill you, they still wouldn't have any right to kill you.
I'd like to see a citation of anyone making something in their free time that a company was able to steal from them based on some work contract. And if someone can find citations that it does indeed work this way where you live, don't just quit. Move away from what ever insane place you live that allows this kind of literal slavery.
The question here is how common it is in twinn's industry and whether it seems ethical and worth putting up with. For a programmer to be barred from freelancing, consulting, or side-projects AT ALL is pretty extreme. But it's probably easier for his company to leave that in place instead of trying to define what is or is not competing with their product line.
Look at it this way: when you sign these agreements are you on equal footing? If every company has them then you have no choice to but sign if you want to work. So you're not freely giving up your rights, you're basically being forced to.
They might be able to give you some grief if you're doing the exact same thing at home as you're doing at work, but even then I doubt they could enforce anything. Didn't Zuckerberg sign some agreements about copying not copying work?
Of course, if they hear about your commercial project(s) they could fire you for it, but if they'll fire you for something like that they were probably looking for a reason anyway.