I would have thought so too (and did), but at my previous employer they took over another company which broadened the scope of the products (VFX software) they offered, which then came into conflict with work I'd been doing in my own time.

I ended up signing a specific non-compete and agreeing not to open-source my stuff for a year for the opportunely to work on something I wanted to there, but it turned out that up to that point, the managers' interpretation of the contract that I thought would have prevented working on open-source competitors (i.e. blender or something) meant that they wouldn't have been able or had the right to. Which I found surprising, but I guess wasn't really put to the test.

So I guess it varies.