Legal framework in the US
The right of expatriates to vote was established by the Overseas Citizens Voting Rights Act (OVCRA) (1976).
The statutes enacted by OVCRA were later consolidated with some other voting statutes by the Uniformed and Overseas Citizens Absentee Voting Act (UOCAVA) (1986). The relevant parts are codified today at 52 U.S.C. s. 20301-20311.
The subsections most relevant to your question are 52 U.S.C. s. 20302(a)(1):
Each State shall permit absent uniformed services voters and overseas voters to use absentee registration procedures and to vote by absentee ballot in general, special, primary, and runoff elections for Federal office.
and the definition of "overseas voter" at 52 U.S.C. s. 20310(5)(B):
a person who resides outside the United States and is qualified to vote in the last place in which the person was domiciled before leaving the United States.
This requirement for states to permit expatriates to vote does not extend to citizens that have never resided in the US. Regardless, some states offer that privilege anyway, dependent on other conditions that vary from state to state.
Justifications given in the House of Representatives
Some motivations for the original OVCRA were given during congressional floor debate on December 10, 1975. (121 Cong. Rec. 39,731-39,737)
We recognize the principle that the right to vote for national officers is an inherent right and privilege of national citizenship, and that Congress retains the power to protect this right and privilege under both the necessary and
proper clause and the 14th amendment. The Committee on House Administration concluded that a U.S. citizen residing outside the United States can remain a citizen of his last State of residence and domicile for purposes of voting in Federal elections under this bill, as long as he has not become a citizen of another State and has not otherwise relinquished his citizenship in such prior State.
The legislation we propose today seeks to insure not only the right to vote in
Federal elections, but also the right to international travel and settlement.
These overseas citizens of the United
States are scattered all over the globe.
About one-quarter of them live in Canada;
about one-quarter of them live in
Western Europe; about 9 percent are In
Mexico; and the rest are in more than
100 other foreign countries. They are American citizens. They pay U.S. taxes.
They deserve the franchise.
I do not think we are going to find
anybody voting under this law except
people who are genuinely interested in
the United States enough to go through the red tape to get the absentee ballot to vote.
I agree with the gentleman from Ohio
that most of the people who would desire
to vote would certainly be people who are still interested in the United States,
who consider themselves to be citizens
and who would vote if they were given
the opportunity to do so in their own States.
The right to vote is one of the most
basic rights of American citizenship, and
I strongly believe Congress has the power
to protect this right under the necessary
and proper clause and the 14th amendment.
The right of international travel has
long been recognized as "an important
aspect of the citizen's liberty" Kent v. Dulles, 357 U.S. 116, 127 (1958), and has been reaffirmed in 1964 in Aptheker v. Secretary of State, 379 U.S. 500, 505. And the right is not just limited to those who are always on the move.
By this act, we will simply insure
the same rights to private citizens
residing overseas, many of whom pay
state taxes and have an active interest
in the conduct of their Government.
Allowing expatriates to vote aims to simultaneously protect the right to vote and the right to international travel.
The assumption is that those that go through the trouble to vote from overseas are genuinely interested in the US.
"They are American citizens. They pay U.S. taxes. They deserve the franchise."