It's been years since I listened to them regularly, but their first couple albums are pretty punk-ish. I mean, it's obviously punk music to me. Sure their later stuff got pretty wimpy and schmaltzy, but even on later albums they did some punk songs occasionally.
I wouldn't call the Beastie Boys last several albums "punk" but that doesn't mean they weren't a punk band.
Also, you're wrong about the punks loving Dropkick Murphys. Those are douchebags, not punks. There is a difference, albeit a small one.