Does society accept this behaviour outside tech? Imagine that a Michelin restaurant would claim copyright on the dishes cooked by the chef in his spare time. Or that the big farm would ask for a piece of those carrots grown in the back yard.
This is mostly a US thing. If I don't implement a DIRECT competitor to my company in my free time, the company can do absolutely nothing about it here. IF I implement a competitor and put it on the market, they can fire me. They can't however, sue me for the posession of the software. That's just a legal nightmare in the US.
I was in a similar situation where I had an idea for a similar product the company I worked for was toying around but was not able to implement. In the end I quit, waited a few month and then implemented it, just to be sure.