Because that court INTERPRETED the laws. They decided that airsoft guns were replicas because they can, it's a Court.
What they decide then becomes binding on future decisions. That's how the legal system works.
Now, again, another interpretation comes from CBSA and CFC; that Replicas can be seen as Prohibited. If that is the way they see it, almost everyone here is screwed.
Even so, with all that stacking up, you really should wait until you are 18.
Remember that what happens to you depends on the Interpretation an Official makes. That Interpretation will vary greatly wether you are over or under 18.
Who will carry the blame will also vary greatly. I dont want to be near that kind of crap just so a teenager can get a realistic looking gun. Not when being mature enough to wait a year or two is all that's required.
Do you want your ass to depend on an interpretation? I dont.