I've actually wondered this too. I remember hearing about these temp towers at the 2013 inauguration. I understand how they allow more users to get a signal, but as you said.....what is the backhaul actually on? It could be distributed through a cell tower you may not get a signal from is my guess (like 1 that's 1mi away), or ran on a fios network (like voip).

http://www.huffingtonpost.com/2013/01/07/cell-phone-towers-i...