It is very common in the US for government entities to build towers (for water storage or to fill a gap in emergency services 2-way radio coverage) and then lease space to carriers to offset the cost and reduce the number of towers in a area. I'd assume economics before spies if I saw a cell sector on a police tower.
Seeing as companies like Harris build equipment for both running and spying on cell networks, it may make sense for them to offer a tower-mounted system...