More Snowy Owl Sightings in the U.S.

It's possible that due to the popularity of Harry Potter some people might seek them as pets, and simply release them when they got tired of them.

These aren't "pet" birds, it is VERY difficult to get a permit to own owls and other birds of prey, and as far as I'm aware you would need a falconry license. It's not like you can run to a pet store and just purchase one.
This is called an irruption year. There were lots of lemmings this past breeding season, that means lots of young snowy owls, and so lots of them go further and further south, looking for food. I know one in central Ohio was found dead of starvation, probably the same fate for quite a few unfortunately.

I also know one showed up in Hawaii of all places, and was shot when it couldn't be scared off an active runway.

That would suck, being "the guy who shot Hedwig" eh?

To clarify my first statement, I'm not sure on the exact legality of owls regarding falconry licenses, since you don't exactly use them to hunt. I worded it kind of poorly though. Eh.

Actually you were correct, you do need both state and federal authorization to possess any bird of prey. Snowy Owls, like most raptors in the U.S. are protected under the Migratory Bird Treaty and can only be removed from the wild for personal use by someone who has a falconry license from a state who's falconry program has been certified by the U.S. Fish and Wildlife Service. The process to become a licensed falconer is quite extensive and it usually takes more than a year just to reach the entry level that will allow you to possess certain raptors. Snowy Owls are an amazing bird that we think could adapt well to trained hunting situations, but not many have tried because they are so rarely seen in the lower 48 states. As a large, diurnal (daytime) hunter, a snowy owl could hunt a variety of game in a falconry situation with the proper training and care. The biggest problem is that by the time a falconer with the proper licensing and interest finds out about a migrating immature snowy owl in his or her area, there's usually already a large gathering of other people observing the bird and it wouldn't be appropriate to trap the bird and remove it from the public's view. However, if a young snowy owl or other type of raptor is removed from the wild by a falconer, it has a very good chance of returning to the wild within one or two years. Since the birds are flown free while huntin, they have many oppotrunities to release themselves and continue life as a wild (more experienced) bird.