In MW2 thousands of people got vac banned for no reason. So yes it could happen. However I haven't seen a vast number of people claim they aren't cheating.
Maybe they used any kind of mod?
However personally I would never defend any CoD player. But sure it's not fair if a player with a normal client gets banned.
And even if it's true what you said, why they play it then? I would never touch a game with a ban rate like this (I wouldn't touch CoD anyway but thats not the point :P ).
Edit:
There have been instances in the past where VAC has been wrong. For instance, they have lifted over 500 bans for MW2 due to complaints, and there was a HL1 mod that would, for some reason, trigger VAC to ban sourcegold on your account despite being a single player mod.
What do you mean with complaints? But the people were able to proof their innocence?