Interesting. I didn't know they were voltage-fed. If it was a standard 1/4 wave groundplane style I always thought the high voltage region was at the top.Also.. since mast radiators can have a pretty high voltage against ground, they need to be well fenced in case some idiot decides to go for a climb.
Also, the fact there are multiple towers allows the signal to be directed in a certain direction by adjusting the phase with which the towers are fed.