can my boss threaten my job without human resources present?

the superintendent of the company i work for has been stealing the companies scrap aluminum and other scrap material for some time now. I was sick of seeing him take the scrap so i wrote on one of the pieces of aluminum he was stealing. It read "reserved for ****** boat gas". One of my co-workers (a snitch) told my boss and he then told the superintendent. A few days later i got called to the office and was reamed out. They told I could not talk to any of my co-workers about the meeting or I would be fired. Then they told me they would evaluate my job on monday. All of this took place without human resources being present or being notified. The superintendent also told me he didn't care what i thought about what he was doing and it was none of my business. Is this legal to threaten my job when i knew he was doing something illegal?