They were, you are right, to those who immigrated not to those who were born in the US. That's the issue here. These people were born there and are being denied their citizenship.
Fine. The Latvians should make a deal with Putin to give Russian passports and residency visas to those who don't want to be assimilated. However if they don't want to be part of the Latvian nation, then they should not expect to dictate how it is run.
Ivan