Imagine how many crimes local police could prevent if they did have access? Unlike finding small terrorist cells among tens of millions of people, busting local criminals is very easy if you can spy on their communication.
They could probably save thousands of children from serious abuse. They could definitely save some people from being murdered.
And yet, it's still not worth it because we would be living in a police state, not a free society.
They are calling for mass video surveillance of everyone's encounters with police.
That's because there are serious problems with police accountability. Citizens benefit because bad police (which we hope are a minority) are discouraged from acting poorly, due to pervasive evidence, and the police benefit because there is a clear documented record of What Happened, which helps prevent spurious claims of police misconduct.
It's something that most of the police seem not to want, judging from reactions towards journalists and people with cameras, so it's not the __police__ asking for more surveillance. (Well, I mean, they are, but not in this way.)
"Mass surveillance is the intricate surveillance of an entire or a substantial fraction of a population in order to monitor that group of citizens."