They've been carrying those for a couple decades.
Exactly,
What many don't realize is that before the current crop of M4's and ARs, the M16 platform was not universally loved. Some of it had to do with the cleaning instructions and the poor propellant used when the rifle first rolled out. There was also considerable prejudice from the M1/M14 crowd. The AR platform of today is not your father's AR.
Now there was a time when the Mini 14 was the preferred platform for the 5.56. It was perceived as being wood and steel, more Garand like. It was also much less expensive than a Colt (which was the only brand going back in the day) and with so much of Colts production going to the military AR platforms were hard to come by and very expensive.
The Mini 14 developed a solid reputation for reliability and cost effectiveness. It became a preferred Agency long arm for those agencies that used rifles. You will still find many in service, the NYPD being no exception.
Mini 14's were in short supply too in the early and middle 70's and a lot of them went to agency sales. Ruger was also getting a bigger share of the LE market with the Security Six and Speed Six Revolvers. Whereas it used to be the big two: it was becoming the big three especially as Colt continued its bumpy ride.
A well armed LEO in the 1970's would have the following if they were in a progressive department:
S&W Model 19 Combat Magnum .357 Revolver
Remington (maybe Ithica) 870 12 Ga Shotgun
Ruger Mini 14
If you had those three, you were ahead of the curve. Actually you would be well armed today if that is what you had.
Don't sell the Mini 14 short. People like to bash them, but for a long time they were the most available rifles in their caliber and respected. I wouldn't feel under gunned if I had one. There are arguably better platforms today but they will get the job done.