Also, we do have technology more sophisticated than metal detectors for security. Xrays, for example. Or patting people down.
If guns can be made out of plastic and we have areas where we want to not have guns allowed, we can no longer rely on metal detectors to keep that area free of guns. Outlawing the plastic guns will mean that outlaws will still try to bring them through metal-detector areas, because the plastic guns can still exist, you just have to be craftier about obtaining one. Making something illegal because it could potentially be sneaked through security is ridiculous. Following that reasoning, anything that can’t go through a TSA checkpoint should be illegal.
Not to mention how many ceramic knives could get through a metal detector, and one could cause plenty of havoc with one of those sharp things, good lord.
Plastic guns are not much better than a BB gun. Plastic does not handle fire very well.