Actually, they were never really legal or compliant, but you wouldn't know it..
Back in the early 2000s one hobbiest group began documenting part 15 AM billboards: https://web.archive.org/web/20081015124256/http://www.netfeed.com/~jhill/1610.htm The comment was made:
"Maybe some one can use this loophole to run a community orientated low power station ?"
The hobbiest who did field reports said the range of these installations averaged anywhere from 5 to 15 miles "more range than a LPFM" one said. Apparently the FCC were taking feild strength readings and passing their installations be because 15.219 has no feild strength limits.. and that appears to have been the case for at least 10 years. Radio Billboards (part 15 AM) was BIG BIG, very big business back then, but the transmitters themselves were never actually for sale, they were only for lease at $2000 a month, and they were spreading all over the US and getting big press... They were considered compleatly legal, and he made millions of dollars off those leases.
Is it legal today? Well, not any less legal than they where back then, but back then the FCC deemed them compliant.. however, any hobby station would get shut down for doing the same thing (as someone had pointed out)
One thing is clear from my own research..
Ground leads weren't even a factor until the early 1970s (which is understandable since that when the part 15 whip and mast were first manufactured) and in 1971 the FCC made it mandatory that Yellowstone and Hwy departments who were using them either ground mount them or disconnect the lead...
However, the issue kind of faded away in practice and by the late 1970s the most common installations were either pole mounted out of reach or more commonly mounted on the roofs of existing structures, often they were tall buildings.. at that became the norm for the next 30 years
The company is actually still in business though dont think they still offer AM, but that's how they started (it was the same guy who originally was only selling Talking Houses as a distributor who had teamed up with Atlantic Records and was involved with their many installations that had got so much mass media focus back then. Anyway, he hadn't been satisfied with the audio quality of the TH transmitters he was marketing and decided to manufacture his own transmitter.. and thats when his multi-million dollar business Radio Billboards was born and thrived for many years and I could find no controversies or citations indicated anywhere concerning their use, which was quite wide-spread.
I had conversed with him about 10 years ago, forgot all about it, need to th touch base with him again as I've got questions I never thought of asking him back then.. l would sure like to know it certification number.