That's a good point, and also, what a horrendous thing to have in your contract. I wonder if it's even legal for them to claim ownership of unrelated code done outside work hours.
I actually check the laws in a US State before moving there just to make sure it this type of intellectual property grab is not allowed. By rights any jurisdiction that allows this should have zero people producing intellectual property, but there's so much friction in moving elsewhere for most people that companies can push it and get away with it.