It's still not that hard for a savvy attorney to introduce doubt about the provenance of a magazine, that's what people are forgetting. The state is compelled to prove that the magazine is illegal. Typically this means another "ex spurt" from their side, usually an anti gun LEO gets thrown on the stand as a witness. Any good attorney could probably rip that guy to shreds. Problem is someone with a shit/low buck attorney, that isn't going to happen, hell they probably won't even go to trial and work out a (likely disabling) plea deal, with or without the mag charges. (chances are if you're facing the system, you're not just there because of magazines. )
The attitudes are "flippant" because MA's rendition of the AWB, from a legal standpoint is a gigantic piece of shit. When the feds had it, at least they had a provisio which basically 100% settled this issue from a standard of evidence standpoint. MA bullshit law doesn't incorporate that chunk of US code which basically said "Any claim of a pre ban magazine is to be taken at face value in the eyes of the law unless the magazine in question is marked as indicated in the law". (not the exact wording, but the meaning is identical.).
-Mike