Is it legal for a state trooper to speed when they are not responding to a call?

I was driving behind a state trooper. I wasn’t familiar with the area but I knew that the speed fluctuated between 60 and 70MPH. I decided to keep pace with the trooper and I set my cruise control to 70MPH. The trooper pulled over into the left lane, slowed down, got behind me and then pulled me over for speeding.

Clearly, the trooper was not speeding to respond to a call because she felt pulling me over was the high priority.