Just as much of a Christmas movie as Home Alone

The way Christmas is used as a way to get the family to leave Kevin behind in Home Alone is the same way Christmas is used to get John to Nakatomi Plaza in Die Hard. It's a plot device used in both movies solely to get characters from one place to another. So if you consider Home Alone a Christmas film, it doesn't make sense not to consider Die Hard one.

Not only that, but films are a form of art. Art is subjective and can be interpreted differently. If people interpret Die Hard as a Christmas movie let them. It's nothing to get your panties in a wad over.