Yeah I was very surprised to read this in my current contract (first employer, the Netherlands). It basically says: "as is the default, we own products/code/whatever that is relevant to our business, even if you made it in spare time, but we make the exemption that contributions to pre-existing open source projects can be under an open license, because we are so benevolent and don't want to discourage this".

I asked about it, but without raising a formal complaint, I didn't get more than the general logic, which is "because you also learn from experiences on the job, it's unfair if you can can put stuff together and then sell it or even give it away for free to potential competitors".

I'm still considering whether or not to make a big deal out of it in future contracts; but next month I'm going to do a master's anyway, so it was a temporary limitation (and I accepted with that in mind).