It actually kind of makes sense. Their goal is to just pack things in as tight as possible. They don't chase the puck at all really, so they're never really in the position where they need to overcompensate with their stick or try and cheat with bad body position which is where so many penalties come from.. The only place they're actively applying pressure is the forecheck but even then it's not that labor-intensive after 1F.
They are perfectly content laying down for 8-9 minutes at a time and letting you pile 30 shots on net and aren't going to try and overcompensate with even a single extra attempt at a pokecheck.
Hit the nail on the head I think. As I've said before, you can get away with more holds and other interference in the playoffs, usually as long as it's not something blatant right in front of the ref and you don't take out a guy who's miles off the puck. The Islanders are pretty good at that, as they should be. It's illegal stick work that the officials are still more willing to call, and the Isles players are
very disciplined with their sticks. Very few trips, slashes, high sticks, blatant hooks etc. They keep it clean in that regard, and that's why they don't get sent to the box very often. The Bruins by contrast drew a ton of stick-related penalties in the regular season, and while they've cleaned it up somewhat in the playoffs, it's still something they need to be mindful of.