First of all, there is nothing preventing them from photographing you if you are in a public area or an area that can be seen from a public area, ie: your front yard, standing in your front window, etc.
That being said, you too can photograph them.
Seems strange that they would do this if you were clean. Unless they suspected you of something and they plan on keeping an eye on you in the future.
They may have had reports of poaching in the area and are building a mugshot book to show to witnesses. That would be my best guess.
Beware, Big Brother is real and working to get us all.