http://dictionary.reference.com/search?q=punkPunk:
1. Dry decayed wood, used as tinder.
2. Any of various substances that smolder when ignited, used to light fireworks.
3. Chinese incense.
adj. Slang
1.
Of poor quality; worthless. 2. Weak in spirits or health.
1. Slang.
1.
A young person, especially a member of a rebellious counterculture group.
2. An inexperienced young man.
2. Music.
1.
Punk rock.
2.
A punk rocker.
3.
1. Slang. A young man who is the sexual partner of an older man.
2. Archaic. A prostitute.
Punk is a very broad word. When one refers to music, they refer to either the rebellious counter culture person or the general sound of it. Translation:
If they're doing something that's rebellious and is countercultural, then the band
members can be classied as punk. Now if the music they play sounds like punk, then it obviously can be punk.
Judging by the sound of older punk bands that I know, and I don't know much about simple plan or sum 41 or good charlotte, sound wise, they're not punk. They're something else, or is at least mixed with something else.
As for them being apart of a rebellious counterculture thing, I'd leave that to you to decide.
EDIT: I agree with valug as well