That's exactly the problem. No company should force workers to have only their job. If full time equates to 8 hours, I might well take another 8-hour job (of course that's very tiring and ineffective though) and work 16 hours a day. Would I personally? No. But should anyone who wants to do it be allowed? Yes.
What about being a parent? From what I hear it's like having a second job or even more work than that.