More likely that they're UNDERemployed, bored at a dead-end, stupid job, so they have time to answer questions.
That said, the typical high achiever on Stack Overflow is also a high achiever at work. I know for a fact that Jon Skeet's employer loves him despite the fact that he has the highest reputation on the site.
That said, there are a lot of working arrangements in the world, and "spends almost all time working on assigned tasks" is just one of them. I get quite a bit of Internet karma (numerical and otherwise) while physically present at the day job, in downtime between tasks. My employers and I have an understanding on this: they treat me like an adult and let me manage my own use of time, I keep earning all the gold stars they stick onto my productivity evaluations.
("Has time to X" is not a measurement of productivity. It is a poor proxy for productivity, which is probably being used because management is not competent at actually measuring productivity. If you actually measure productivity, and the numbers say that Engineer X is 40% more productive than the company average, and you think "That's funny, he seems to have time to update his blog and check Facebook in between reading on new Javascript techniques and, oh yeah, there might have been some coding involved", then you know what that tells you? That Engineer X is 40% more productive than the company average.)
I cannot tell much about my current job's details (I'm bound by an NDA), but mostly it involves not coding as such but rather answering the other people's programming-related questions, in other words exactly what I'm doing on Stack Overflow.
I also have a blog where people ask me the questions and I answer them.
SO and the blog just help me to be in form for the job: almost any problem the people contact me with at work I have been asked about (and answered) a dozen times on SO and the blog.
Might be worth proving it wrong.