This is insane. There are only so many hours in a day, and I can't imagine being motivated AND capable to re-implement anything beyond a trivially simple application in my own free time. If you have the ability to create, I think just carbon copying something from work is too boring for 99.999999% of people (stats made up). The capacity for shitty employers to do shitty things is much higher than an employee, due to their greater resources. I would also wager that a greater percentage of companies than people are willing to rail the other if given an opportunity; profit motive and all that.
At a previous job I signed an agreement giving all IP in a _very specific domain_ to my employer, regardless where it was created. I was okay with that. If I build a missile defense shield in my garage they can have it.