It wouldn't be, and that is what was made clear years ago. While there is any doubt as to the potential legal implementation of the law that permits only 22LR in semi auto, the public has the reassurance that possession of a sub 12 semi auto rifle will not of itself lead to a prosecution.
There also happens to be law that requires every London cab driver to have a bale of hay on board in case the horse needs it. That has never been repealed, but I don't see too many cabbies in dock over it.
Why hasn't it been repealed? Because it isn't a problem and doesn't warrant the time being spent on it. Exactly the same reason why the semi auto law wasn't clarified or altered, it didn't justify the effort, and the advice that no prosecutions would follow was adequate in the circumstances.