Even if it's not legal, I'm not going to sue my employer to stake ownership over an evening project.

You don't need ownership here, just authorship.

Even if they're not likely to win in a court battle, most people don't want to risk a dispute over something trivial.

Would you expect a little game jam game created in a weekend to lead to an ownership dispute with an employer? - Probably not, but I'm aware of a case where it has happened...

American exceptionalism at its finest.