It's to avoid double processing, the bit that counts is forum_make_mail_post() which calls format_text() with the type of input that was up to the author to decide $post->messageformat . This then gets passed into the event data using FORMAT_PLAIN so it doesn't get processed a second time.
Re smilies check what filters are turned on /admin/filters.php see:
https://docs.moodle.org/28/en/Display_emoticons_as_images_filter