That assumes the world is zero-sum. If my employer has a code base that I find more enjoyable / fascinating than any of my side projects, maybe the employer does get extra out of me from me putting in extra hours, but I also get more out of him by both delivering what's expected of me and getting to toy around in a code base that I find fun. I might even be improving my earning potential (if the "fun" argument isn't good enough for you) by building skills working on that production code base that I wouldn't by just messing around on side projects. The dynamic is obviously very open to abuse and people have to protect themselves from that, but I don't agree "extra hours = sucker" is universally true.

I like building stuff. During the day, I get paid to do it for my employer. In my off time, sometimes it takes the form of a personal project. Sometimes it takes the form of a house or garden project. And sometimes I’m still thinking about something that I’m enjoying at work, so it takes the form of a little more work on a work project. If I’m enjoying it and I’m not doing it to the exclusion of family time, then whatever, I’m happy.