I hate to talk about correlation/causation, but there's typically some significant demographic differences between profs with and without tenure.
My experience is that tenure-track profs were a heck of a lot younger, meshed well with the students, hadn't spent the last 20 years teaching the course, and were more likely to put in more time and effort on the material. Tenured profs also tend to have a lot of things sucking their time (obviously researchers, but department heads and/or deans are worse), so they dump a lot more on the TA's and are pretty tight for office hours.
I'd be curious to see how things break down when they account for demographic differences. If that's even feasible.