When you send the XML as an attachment, Outlook launches InfoPath inside of the message body. So you're seeing InfoPath running and you can fill out the form from Outlook.
When you send just the active view, the XML is converted to MHTML and buttons are stripped out since they won't work (no InfoPath business logic going on with plain MHTML).
The preview window of Outlook doesn't host InfoPath.
Hope that answers your questions ...