It was a little thing, but it bugged me with CommunityServer.
Of course the functionality was as designed, and (of course) there is a way to fine-tune it. In this case in the <markup> section of the CommunityServer.config - it allows you to specify what tags and parameters are valid (and any not on the list simply get converted to safe markup. It's very clever. And just goes to prove I really should try and find more time in the day to read manuals!