I've never heard them described as pop punk. If you know anyone in real life who has described them as such, they don't know what they're talking about. And no, Rancid, definitely isn't pop punk either. It's not my opinion.
http://en.wikipedia.org/wiki/Pop_punk
Weezer is power pop. Nada Surf is alt rock. Rancid is just punk in general, I guess.