I've now looked at Wealthfront and their site says "We cannot support clients residing outside of the U.S., including U.S. citizens residing abroad."
Betterment seemed to care the least amount, but they still say "The products and services provided under the Program by Betterment, Betterment Securities, and the Website are not being offered to, and are generally not available to, anyone located outside the 50 U.S. states, including U.S. citizens residing or working abroad." However, this seems to be more from a standpoint of limiting their legal liability not that they don't want to work with U.S. citizens residing and/or working abroad.
In conclusion, I can't find one large reputable financial institution that is happy to open an IRA account for a U.S. citizen living and/or working abroad. Many say it's related to the Patriot Act or some other regulations. That's fine, but at least according to the IRS, U.S. citizens abroad can contribute to an IRA if they meet the IRS' requirements, so all the above is annoying.