Maybe because I can only think of 2 .177's with a power output low enough to qualify, the Gat and the G10.
The Gat the is mitigated by the fact it in no way imitates a real firearm.
However, the G10 does...but the fact that there's not exactly a rash of crim's using them to hold up banks or the fact that theres many thousands of pre-VCR airsoft replicas out there in secondhand circulation probably mean that any prosecution is hardly in the public-interest.