The problem of "push polling"

I just stumbled upon this article (here--Tales of a Push Pollster)linked from a "gay" website, about ccAdvertising and was intrigued by the accusations about "push polling."

I've often heard about "push polling" but can't say that I've--especially recently, taken the time to listen to purported examples of it. But this assertion:

"one of the dirtier, yet mostly legal, tricks in a political operative's bag of last-minute campaign tools."

got me interested and one example was for the Ford/Corker race in Tennessee, so I checked it out.

Sorry, but what's "dirt[y]" about this and why shouldn't it be "legal"?