Who is not registered?
Anyone who has not been issued one of the documents designated as evidence of registration under 8 CFR 264.1(b) and has not submitted one of the forms designated at 8 CFR 264.1(a) and provided fingerprints (unless waived) is not registered. Aliens who have not registered include:
...
Canadian visitors who entered the United States at land ports of entry and were not issued evidence of registration; and,
This does seem to imply that Canadians who entered not at a land-crossing may actually be registered. Though it's not clear why. But if they aren't registered why does this explicitly call out visitors who entered at land ports of entry??
So apparently every snowbird is currently a law-breaker and there's no way for them to have complied with the law. What a well run government this country has.