I hope it's an April Fools thing, because ,sure, maybe they were annoying someone, and you should really pick up on the signs and know when to quit and move on, but Improv Everywhere have just pretty much been doing fun things in public, and making hundreds of people's days a whole load better with every event they pull.
I seriously recommend looking up their stuff, the Best Buy thing was great, where they had about a hundred people show up in the staff colours of blue n khaki, but they all had to act like they didn't know why people were asking them questions.
There was another where they took over a whole set of listening posts in a music store, and did some choreographed dance moves...then just left.
Of course, the 'no pants' subway thing is a regular and a great idea. I'm a cynical, miserable bastard at the best of times, but even I can see there's not an ounce of malice in these guys, they just want to spread some fun and happiness in a world that's sorely lacking.