I'm self-employed in Germany, and when I was employed, what was in the contract went more in the other direction: it was explicitly allowed to contribute to FOSS projects. Of course, it still would not have been OK to randomly add GPL software to a closed source customer project. If anyone had been stupid (uneducated, really) enough, somebody else on the same project would have noticed.

Yeah, the second sentence is what I meant. This is why I'm writing comments on Hacker News rather than legal contracts :)