you have history being disrespectful with staff and i do believe joking about cancer could be offensive to some people.
Anything you say could be taken as offensive. You could get offended about this comment, but its a risk I am willing to take to change your mind in pursue of a fair action.
Instead of saying it could be offensive I would describe it as an action aimed to offend a staff member as that it was
here is an interesting piece regarding this from a person I respect very much. (its less than 2 minutes you have time to watch this)
you are correct everything could be taken as offense but a joke about cancer is extreme and disgusting. it was a valid punishment as he joked about something that could kill you, an extreme disease. what if someone in the server had a history with cancer? they would get offended it is like the racism policy no matter what race you are it is a racial slur. in this case no matter if you have cancer or not it should not be joked about since it's extreme.