In my experience, at a small liberal arts college, there is little motivation for research, so even if the professor is teaching four or five courses a year, it is possible that they aren't doing much outside of the classroom work. Thus, tenure is a ticket to an easy life of teaching the same material year after year, with little value or motivation to publish.

I don't disagree with your point, however, and I had professors that did work hard outside of the classroom on their research, and certainly, in the case of larger universities, many do work long hours.