Can salaried employees be docked pay for not signing a write up?

I submitted a letter of complaint to my boss/owner about the improper use of the N-word in the office as well as a complaint that I was singled out and punished for using my cell phone when my child called me at work.

My letter pointed out an observation where I heard the owner and his son use the N-word in a nasty way towards a person; i "observed" a Sr. GM throw away an employee application from a young black man and on a weekly basis myself and other staff observe the owner's son walk the halls of the business using his cell phone and actually play video games on it.

Today, my boss/owner asked me to read a letter to me that had untruths about things I have said and outright lies about me and the letter of complaint. In a nut shell, the letter would make it seem as if I were to recant my letter of complaint.

I told my boss/owner that I could not sign his letter to me because there were untruths in it about me and my letter of complaint. He exploded with name calling and a very violent outburst of aggression towards me with foul language and the throwing of papers and slamming his hands on desks.

Evan going as far as calling me very vial names and mocking me like a 3 year-old child might do. Likewise, calling me a lair and saying that he hates me.

His reaction was to tell me that if I didn't sign his letter he would suspend me for insubordination for 3 days without pay.

Can he do this if I am salaried and work over 40 hours a week?