Employees don't have any control over what you write on your own time. I think some older IBM contracts claimed they did, but if you sign one of these today, you're an idiot (and they're not enforceable).
I don't think this is entirely true, at least in most states in the USA. It's called "duty of loyalty" and it basically means you can't compete with your employer. Whats competition is gray but I'm sure you can imagine examples of shades that are clearly competition, shades that are clearly not, and all kinds of shades in the middle.