"Does that make them the origional plagerists?"
That's like saying the NFL plagiarized MLB by using big screens. You are talking about the same people. Christianity was a religious movement, not a foreign invasion. The Pagans and the Christians were the same people. You can't plagiarize yourself. BTW, Halloween has nothing to do with Christianity. It is not Christians who celebrate Halloween, in fact the church has many times asked people not to participate. Halloween is a sectarian celebration that mimics certain aspects of the winter solstice celebration of centuries passed.
Maybe I'm wrong but I always thought the point of Halloween was for kids to dress up as characters that they thought were scary so as to make them less scary. If you think Obama is scary then dress up as Obama on Halloween. Then he won't be so scary, at least to the kids.
Why complain about holidays? Just enjoy.