Is profanity in a courtroom or magistrate office illegal? or just rude?

In trying to deal with very rude people in a local magistrate office, I became very frustrated, sighed and cursed softly under my breath. This wasn't directed toward a person, but just a manifestation of frustration that wasn't meant for others' ears. Unfortunately someone heard and I was threatened and told I couldn't use that language because it was a "courtroom." Is that just a rule of curteousy, or is it a punishable crime. Either way, particularly since it wasn't directed at the magistrate or his rude employees, I can't see how it was a breech of any rule or law. I have a real problem with the loss of first amendment rights over a minor speeding ticket (which by the way I don't believe is warranted, but don't see a point in contesting, since I'm moving and won't be able to come back for a court date without much expense on my own part. I've never had a speeding ticket before so this was all new to me).