I haven't read enough to be fully sure, but I think the NFL has seen a bit of a change on officiating over the last 40 or so years. It used to be that these guys were judges, CEO's, etc. Basically, older guys that had proved they were ahead of the pack in brains and work ethics via the business world. They also had a passion for football and serving as a ref not only massaged their ego's but was something they enjoyed. Officiating was kind of a hobby for the well to do. Now with instant replay, home recordings, etc., there comes the need for the job being done precisely and with as much accuracy as we at home can do it frame by frame. Refs are required to have more training and are more scrutinized then ever before.
So while it may not be a full time job in the sense of 40 hour weeks, I think both the men and women that are doing it are much better trained then 40 years ago.
Should they be paid like Football players? No way. But they also have an important enough job, in an industry that makes billions, so the owners have to balance supply and demand with quality and pay.