Wrong is what feels wrong.
At any job there's a line between acceptable screwing of the company and fraud. It's nothing you can define, but it's there, and most people would place it in roughly the same place. It's what feels wrong. Browsing the web when you should be working (and there's work to be done) is technically stealing time from your employer, but no-one would think it was 'wrong': just don't get caught. Having an entire full time job that you're doing in parallel, is on the other side of that line. I can't explain why, but it feels wrong to me.
I'm comfortable with the example of the guy who automated his job and played games: he's available if needed. That's just efficient use of time. If he'd had another full-time job that he couldn't put down the moment he was needed, I'd feel differently.