Places like Guam and Puerto Rico have a vote in the primaries because the Democratic party is a private organization that determines the rules for selecting its candidate. There are people in Guam who are members of the Democratic party, so the party feels those members should have a voice in selecting its leaders. The key words are PRIVATE ORGANIZATION. Dismissing Guam's right to participate in the primary process because they can't vote in the general election is like dismissing the rights of Washington D.C residents who are a member of the NRA from choosing that organizations leaders (key to understanding this analogy is that, at present, it is illegal to own most firearms in D.C.).