If they want to write a note fine, not necessary for the police threat though.
I am a white male, I see and know many African Americans that experience issues just because of the color of their skin and it make me sick to my stomach. No one deserves to be treated differently.
For Richard to write a response like he did, I assume he has been treated differently because of his skin color. Maybe there is a history or previous actions of his neighbor that would cause him to write the letter?
Its easy for me to sit hear and say he was over the top or should not have written the letter, but i have never walked in his shoes. He has every right to respond the in the way he sees fit.