Why does WordPress delete iframe code?

With Tinymce installed, the Editor keeps the iframe tag until you publish it, then it’s gone.

I’ve experimented with the tag, keeping it in both ‘preview’ and source mode and even downloaded the post to MarsEdit in an attempt to ‘reverse’ post it to see if it makes any difference.

I’ve read elsewhere that you shouldn’t switch between modes as it screws up the code but for the life of me I can’t figure out why this should be so?

I’ve always switched between modes when editing posts and yes, it puts in those pesky spacers but by and large it left most existing code, functional, so why delete it?

B