Rule
6.2.3 (2) Magazines of the standard service 20 or 30-round box magazine dimensions
must be attached during the firing of all courses and in all positions.
A 10-round magazine with the same external dimensions as a standard
service 20-round box magazine may be used. A dummy magazine with a
ramp for single shot loading may be used if this magazine has the same
external dimensions as the standard service 20-round box magazine.
Standard service 20-round magazines are straight, without curves. Standard
service 30-round magazines are curved.
So you're the bad guy for informing them.
I do the same as you and let them know, but I just let it slide if it's a club match. You are actually doing them a favor, since if they have any brains they will swap out mags before they go to a big match where someone might gig them on it.
B