They are text files, but a text/plain MIME type doesn't let the computer downloading know that it's a BITTORRENT text file, so it treats it like a text file, and not a Bittorrent file. I.e. the server is not configured properly to serve torrents.
This happens often to me whilst using various BT sites. Workaround is to save the file to disk, then point BT to it using --responsefile.
I agree, it's very annoying when I visit a site and I have to download the .torrent file then launch it manually. It's better when the server sets the mimetype to application/x-bittorrent, then my browser knows to just open bittorrent on the file, with only a single click on my part.
It's just more convenient that way.
