On what legal basis? If you start going after the employers for compensation whenever an employee does something illegal in their own time, you're going to completely devastate the economy, and incentivise just batshit crazy employment contracts / working environments.
Follow the behaviour you're talking about through to the logical conclusion.
If your employer is legally liable for things you do on your own time with your own equipment, and required to prove that they weren't involved and that their equipment wasn't involved, the only option that will be open to them is to monitor you every single moment of every single day.
Anyone that refuses to agree to those terms doesn't get a job. You won't be able to say "Well, that's okay, I'll take a job with another company" because it'll be true for employers all the way from your mom and pop corner shops up to large multinational corporations.
