like what? in this case, these are mostly beat cops doing subway checkpoint duty - they mostly write summonses of one kind or another most times. even if they aren't checking bags, having a presence there and simply looking for suspicious people is more beneficial then flagging someone for parking 10 minutes too long at a meter.
"like what?"
Anything that doesn't have close to a 0% chance of catching a terrorist in the midst of a plot. If there's nothing useful for them to be doing on the subway, station them at a different potential target where security is needed.
Hell, if all of the screening forces combined managed to stop one mugging while on patrol somewhere they'd accomplish more then they will all year digging in people's bags.