Because maybe he was under investigation without his knowledge, and taking away the tools used to do his job would have been a dead giveaway?
There's a fine balancing act with law enforcement systems, between giving people the tools they need to follow leads, and them being able to use them to look into things that don't concern them. It's hard to completely stop the second without hampering the first. Decent monitoring systems may help, but then they end up being a pool of data in their own right, and the question then arises about who should see them, and who watches those people, ad infinitum. In the end, the entire system has to be based on trust at some point.
At the end of the day it comes down to professionalism vs temptation - you know you shouldn't be looking, but it's just sitting there asking you to take a peek. There are a lot of people working for the various police organisations across the country, many of them not officers, but civilian staff. They need the access to the databases to do their jobs, but they are all also human, and like all humans, can be good 'uns or bad 'uns.
I'm sure rules and procedures are in place, and it may well be the procedures that are in place that means the number of people being caught is going up. Just think what it would have been like in the '70s and '80s when any safeguards that there would have been probably consisted of a password written on a post-it.