Can I be terminated for posting my resume on Monster?

I was recently written up for posting my resume on Monster.com. In the write-up my employer stated that if I did not remove my resume immediately I would be terminated. I had to sign the write-up, because again, if I did not sign it I was told I would be terminated before leaving the meeting. There was no policy in the employee handbook at the time regarding resumes being posted on Monster. When I had time to gather my thoughts I asked for a copy of the write-up and was told that it was destroyed immediately following our meeting and thus they could not provide me with a copy. Five months later they revised the handbook stating that employees cannot post resumes on Monster. Was it legal for them to threaten my job because I was seeking other employment? Can they dictate whether or not I can post my resume on a job search engine if it is now included in the employee handbook?