Signage on Reefers?

Recently took a driving trip up to TN. and back. I was taking notes on the refrigerated trailers traveling on the highway. I noticed about two thirds of the reefer trailers had Signage with info on the Reefer Carriers that were hauling the freight. The other 1/3 of tractors and trailers had little or no signage on the tractor or trailer. I was wondering if some one could clue me into what might be the story with this group of reefer? I would think they would want to display some form of advertising?