They can't charge you for NOT committing a crime.
Sure they can: if the cop can't tell whether you're committing a felony or not, he's within his powers to toss you into a cage until a judge can sort it out.
And yes, that's the issue at hand: mere possession of a >10-round post-'94 mag in NY is a
felony, while possession of a >10-round pre-'94 mag is perfectly legal ... and generally there is no way to tell the difference on the street.
In some cases, it's obvious:
- Mag is marked with date of manufacture
- Mag type didn't exist pre-'94 (say, 12-round G26 mags)
in which case the issue can be promptly cleared up (though perhaps to your detrement)
In some cases, it's far from obvious:
- Mag is unmarked but looks new
- Mag type was commonly & legally manufactured both pre-'94 and post-'04 (out-of-state, federally legal)
in which case a presumption of "illegal" may be reasonably (legally) made and the arrestee obligated to show proof of legality.
In NY, relevant laws are largely based on "it's illegal, with exceptions". Ergo, if your 30-round AR mag or 17-round G17 mag is in good condition (looks like it's not 14+ years old) and otherwise not obviously pre- or post-ban, it's presumed illegal unless you can prove it's an exception.
I don't miss NY laws.