This reply doesn't come from Showmasters itself,but all convention message boards I know of have in their terms & conditions a line stating that it is not acceptable to promote other events on their message boards,unless with prior agreement with the event organiser,whose board you are using.
This is not a legal issue but is considered as 'poaching' possible attendees to a rival event.
But 'poaching' guests from rival events is considered to be quite ok.
As for it being unacceptable to mention or promote other events, if anyone reads the terms and conditions of using this board, it clearly states that Showmasters are not responsible for what is posted here.
So they can either change the terms and conditions, or they can choke on their cake which they seem to want to be able to both have and eat at the same time.