I'm honestly curious why non-states have primaries? I've got nothing against statehood if they want it, and in that case representation would obviously make sense. But as territories? - Kos
As someone who grew up in the US Virgin Islands, let me attempt a non-scholarly answer:
Representation means a lot.
For a more rounded exploration, take the jump with me.
I currently reside in Norway, where, as a US citizen I continue to have the right to vote in state and national US elections via absentee ballot. The catch is that I have to have resided long enough in a US state prior to moving abroad.
Since I lived in Berkeley, California before moving here, that is where I vote by mail. Had I resided in The District of Columbia, American Samoa, Guam, Puerto Rico, Northern Marianas, Baker Island, Howland Island, Jarvis Island, Kingman Reef, Midway Islands, Navassa Island, Palau, Palmyra Atoll, U.S. Virgin Islands, Wake Island, Micronesia or even the Canal Zone, (which was still under US control during my lifetime), immediately before moving abroad, I'd be shit out of luck.
The residents of these territories and possessions are full US citizens in every way except as regards federal office and federal income tax. Several of these territories have delegates to Congress (the House of Representatives) who have committee assignments in committees and sub-committees and the rights to vote in committee, but are not full members of Congress and thus cannot vote on actual legislation or any other vote on the floor of the House.
The US constitution makes it very clear that only states may send full members to the House and Senate.
So why don't these territories and possessions become states? Several reasons. Great majorities of most of these colonies register and vote for Democrats. Most have populations too small to consider statehood. Puerto Rico and DC being the notable exceptions. With nearly 4 million inhabitants, only 26 states would be larger than PR in that regard. DC weighs in at about half a million. The USVI and Guam have populations of 108,000 and 173,000 respectively. The Northern Marianas have around 85,000. The rest have populations under 70,000.
We're talking about a total of around 5,000,000 US citizens who do not have real representation in Washington.
The path to statehood varies in terms of realism and approach amongst these. DC wants it but Republicans don't because of demographics. This angle would apply to most of the other colonies. Puerto Rico has large minorities that want statehood, but after three referendums on the matter neither independence nor statehood proponents can convince enough of those who prefer the status quo to change their minds. The fact that Puerto Ricans shot and wounded 5 members of Congress in a real act of domestic terrorism back in 1954, would also make an official application for statehood an unsure deal. Congress's memory is long in such regard.
So why do the national parties allow territorial participation in the primaries? It's an attempt to bridge the gap of under-representation in national politics. And there is no constitutional prohibition against the practice. These colonies are still sources of contribution to the party coffers and it is a way of reaching out to US citizens who must follow federal law that they otherwise cannot influence in any direct way beyond the less than influential voices of their delegates to Congress.
To round this off, let me add that as far as the USVI is concerned, and it may very well apply to other territories, this lack of representation allows the territory to collect what moneys normally would go to Uncle Sam in the form of income and most other federal taxes.
Finally, I think it highly ironic that US citizens living in non-US countries can vote for federal office, while those under the US flag in the colonies have no such rights. In my opinion, this should be explored in more depth.
Note: anyone of the citizens of these territories, possessions and district can at any time change their residence to a state by moving there and instantly have federal representation.