So my understanding is that if you have purchased airsoft after 1998 you have committed a crime, possession of a prohibited device. Albeit, you will probably be left alone by the authorities if you are a responsible owner. In addition, the the government most likely does not have the resources to go after everyone that has purchased airsoft after 1998. None the less, you have committed a crime, and could in theory be prosecuted. Just because it hasn't happened yet, doesn't mean it won't happen in the future.
It may be hard to prove that your gun was purchased after 1998, but if asked, I am not one to lie, which seems the only way to make owning a post 1998 gun legal.
So there really are two answers to the question, "Is airsoft legal to own?"
1. If you purchased it before 1998, yes.
2. If you purchased it after 1998, no.
MODT - Magnus Operator Development Team - tu fui ego eris