Adjuncts are a different story.
I taught as an adjunct part time at a state school where professor made $150k. I made $5000 per course. I did it more of less for fun because I like teaching. Which is not bad for extra pay on the side but terrible if it is your only pay.
Edit: as a side note, the department head told me she fought to have adjunct Comp Sci professors make more because the field is in such high demand. I am sure English adjunts at the same school make far less.
That's for adjuncts though. I think for full-time professors they have PhD requirements per the accreditation boards that they need to maintain. But once the quota is met they can hire anyone.
The article gives a median income of under $50K for non-adjuncts. Of course, that includes assistant/associate professors.
I doubt it is because of associate professors. More likely it is because of the size of the school, location, and type. I bet there are a lot of community college students in there.
A quick look in my area (New England) shows associates starting at about $60k and going up from there. But New England has a high cost of living so it is all relative.
Tenured professor at some small liberal arts college usually isn't doing as well.