My best guess is this is a business decision based on the time, effort and cost to the broker that is incurred by their regulator, FINRA. With us, a small introducing broker, during our exams from FINRA they is a lot more time and focus on foreign accounts then domestic.
I highly doubt this is a FINRA issue.
Yesterday my aunt received a phone call from Citibank demanding full disclosure of why she had money wired from overseas, even though the funds were already subject to repatriation and the foreign tax was paid, AND she has been compliant with FBAR. You can read trading blogs where clients who have assets overseas in various banks are all being told they must report everything to the home country. Also, banks are now asking for your social security # if they determine you are a U.S. citizen, so they can report the account to IRS (the de facto collection agency for the Fed).
The OP who received a letter of account closure isn't the only one. Clicking on the link provided by stevegee58, this is also happening to non U.S. customers at Scottrade. The guy posted a screen shot of the letter saying his account would be closed.
Fidelity sent out notices that as of February this year, cash sweeps MUST be placed in a new, government backed cash reserves fund, rather than a regular money market fund. "Fidelity Cash Reserves" have now become "Fidelity
Government Cash Reserves", lol. The reason being is the original money market funds had a new rule that they can "halt redemptions" in case of "market volatility" lol!
The regulations regarding FACTA and FBAR are creating chaos among foreign banks and institutions. The dollar is a fake/fiat currency that has been artificially inflated by the Fed, and my guess is there are back door discussions on things that only insiders know about.