Funny story but that’s how the patent law works in Germany and I believe at least Japan aswell. I don’t know about other places but I wouldn’t be surprised if this applied to the majority of the industrialised world.
If you come up with an idea that could be patented while employed, you must to tell your employer about it and offer it to them. In return, they must either register it themselves and give you an appropriate compensation or decline ownership of it; allowing you to register it yourself.
Rationale behind that, if you work in i.e. IT and invent an IT-related thing after you’ve clocked out for the day, you probably wouldn’t have had the idea if you hadn’t spent the majority of your day working on the topic for a couple years.
I think this is actually quite fair as, even if the company decides to keep it for themselves, it’d register, use, license and defend the patent for you (for a great cut of course).