He's not trying to "scare" anybody. Just to tell people who aren't eligible to vote - i.e., immigrants -- that they will be breaking the law if they vote. If I tell you that you shouldn't run a red light - or you could get ticketed - am I "trying to scare you"? By your logic, I guess it's an exercise in intimidation.
Immigrants are allowed to vote! ILLEGAL immigrants cannot! Telling a person that can legally vote that they cannot is wrong. His inclusion of LEGAL immigrants as a distinct group in this letter is a problem and wrong.