It is my understanding that if you want to filter HTML for blacklisted
elements, then you would use filter="safehtml". There doesn't seem to be a
filter for allowing whitelisted HTML. In that case it's filter="raw" or
nothing. In the CMS the com_content extension has its own filter that is
applied to HTML, and the JForm object itself has an editor item with filter
set to raw.
I think this is behaving exactly as it should be. I don't think this is a
bug, it's a feature request. I think maybe you're looking for something
along the lines of filter="extendedhtml", which would allow some but not
all of the blacklisted items. I know that in my experience, safehtml is too
restrictive in that it doesn't allow iframes which have become a frequent
and easy to implement feature of social networking buttons and widgets.