I was surprised as well when my friend told me the department had bought them, but I saw them with my own eyes. Definitely get the feeling that price was the major factor, but I guess a budget AR is better than no patrol rifle despite the fact they still have a few US Government M16A1s. They also have a handful of Griffin ARs which I had never heard of but was a local manufacturer and I'm sure that had a lot to do with it. All that said, my friend (who is one of their firearms instructors) has said they have run just fine although I doubt they will likely ever see high round count.
I purchased an early M&P15 which may or may not have had some of the STAG parts that Smith was reportedly using while they were getting their AR line fully up to speed. It came from the factory with an out of spec lower receiver which required a trip back to the factory to get an replacement. After that - it has been a great gun giving me years of reliable range use as well as a CSAT class. I have since moved to fancier ARs but I still count it as a rifle I can trust.
My feeling on M&P and Sport models versus other budget ARs is the fact that given S&Ws place in the market a lot of their rifles get into the hands of LE and security contractors while other budget lines like PSA do not. So while the PSA guns may very will be as good as the M&Ps, they don't have the track record. They also may or may not be in the AR business in 20 years while it is probable that S&W will (not that that really matters with an AR anyhow).
FWIW