Quoting from An Inside View From a Google Employee

The thing about Google – and any large company focused on skilled employees – is that everyone there knows that people will spend time off-work thinking about work. It’s inevitable. You’re lying in bed and your brain wanders onto a problem you’re having at work and in half an hour you’ve solved it.
If you’re spending time away from work doing work stuff, why would people object to the converse? We’re all adults here – we don’t have to, or want to, be micromanaged to death

Unfortunately few companies understand that.