I believe they do it under the guise of having a "custodial duty" to protect the assets of the government....
You'll find most police orientated Facebook pages are constantly monitored so they can control the direction of conversation to closer resemble a badge bunny chat room, glorifying the antics of the government sanctioned, equipped and trained commandos. As you might know, people just don't have the self restraint required to keep themselves from asking those oh-so-hard-to-answer questions, such as:
"Why did you shoot Mrs. Johnson's Yorkshire Terrier?"
"Why did you beat that deaf guy?"
"Why did that cop call my mother all those racist names when she was videoing them tase and arrest the neighbor for washing his car?"
I also believe social media is viewed as propaganda venues some police can exploit, and to do so efficiently, they need to dominate and control all activity, thoughts, conversation. A kind of "constitution free zone" if you will.