I am trying very, very hard to come up with reasons to justify this practice. Not much luck.
1) immediate rapid-reaction capability to live incidents without having to lug gear into the subway stations or maintain armament lockers which could themselves be compromised
2) familiarize tactical teams with their intended operational environment
3) put players into "routine" play to act based on specific but closely held operational intelligence (i.e. a specific cell or threat)
4) rifles provide ranged and high-power rounds to put down suicide bombers, etc. from a safe distance . . . also can conceal a countersniper (much more highly trained at accurate rifle fire than your average SWAT team member)
1) terrifies the public, increases fear level, "terrorists are winning"
2) intimidates members of the public from approaching police and advising of observations, suspicions, etc.
3) routinizes guys in black fatigues and gear carrying shoulder weapons
4) risk of negligent discharges in a high risk environment (yes! those of us who train know that NDs are notoriously common among peace officers)
5) small risk of some nut taking away a rifle (yes, kids, rifle disarms are possible) and killing people with it prior to being neutralized
6) in an arrest and control situation, too many rifles means someone out of commission hanging on to the spare rifles (you're not going to make some SWAT officer sling his MP-5 or M-4 and handcuff with a deadly weight over his shoulder, are you?)
7) gullible people believe that a police state is at hand and the fabric of democracy is further unraveling -- this already happened, the rifles are just a difficult to ignore proof
1) not worried about police autofire, the SWAT people train and are disciplined enough to use single shots and short bursts only in response to a deadly threat; any round they intend to fire, I am not worried about
2) not capable of opposing "hidden" terror cell use of the subways, surveillance, etc. because everyone is too busy staring at them including the terrorists
I hope they are thoughtful enough to carry purple or pink balaclavas or some other hidden but quick means of distinguishing themselves from tangos who show up wearing basic black and carrying heavy.
Having the gear already in the stations could be accomplished much more easily and discreetly with arms lockers in non-public areas. Tac teams can do walkthroughs "without" their gear quite nicely. There are small concealable SMGs out there if they don't feel safe walking the subways (you know, like average New Yorkers).
I have to admit that this is security theater unless it's Federal money at work. Then we can call it "Good Training" until the money runs out, at which point it's back to real police work.