They do this with everybody. It's how they do business. They have a long history of No or Slow payment in this area.  Searcher beware. The fact that they would threaten you for telling the truth about them speaks volumes about their lack of character in general.  

I've had problems with them also in the past for non-payment. I finally cut them off for non-payment, and they tried to get me to do more work by blackmail. They told me they had a check made out for their past due amounts ready to send IF I would do more work for them, ..if I didn't do more work they would hold my check and not send it for another month or two. They finally did send me my money. 

I