Philip Morris, Inc. is too stingy to remove signs
These signs required maintenance. See the book The Verse by the Side of the Road : The Story of the Burma-Shave Signs and Jingles by Frank Rowsome, Jr. These things were a cultural icon. They now remind us of a time when things were slower and which we imagine to have been better days (conveniently forgetting all the things that made them the Bad Old Days, a subject which has already been covered here). I would suspect that the land owners, mostly farmers, keep the signs up themselves. The roads that CapK is following, as any of us who have been following his excellent travelog will attest, are roads less traveled now that we have the Interstates. The advertising value of the signs, even if the product does still exist, is minimal, but their nostalgia value is high.
