Yeah, thats where the semi-grey area is right now: states that legalized marijuana don't care, but ATF and feds continually point out its federally illegal and if they catch you its still a big no-no and revocation of your rights when they do.Except the form says an unlawful user of illegal drugs. If they aren't illegal, it's not unlawful to use them.
However, it wouldn't take long for them to realize its a pointless question to include "marijuana" specifically if it were federally legalized and likely update that form and remove certain words.