The whole idea of the SR-71 was that it went so fast that SAMs could just barely not catch up to it before they ran out of fuel. Any other plane couldn't outrun them, but the Blackbird? Well, Mach 3.4 will give you some breathing room.
Missiles and radar tend to operate in a "bubble". Even with the Russian S200 system at 190 miles range, the time you spend in that "bubble" when you're at 70k ft is not often long enough to do anything. That also assumes the aircraft directly overflies the battery. You can play games like pointing the camera sideways and observing from an angle if you know there are going to be angry people with guns nearby.
I imagine thought a little snake left/right might compromise getting the images that was the point of the mission.