I can get that also. But it isn't their private e-mail. That is a public group posting. Like a post to the message board. The way Yahoo! Groups work is that with your Yahoo! profile and e-mail address you sign-in. The message board posting thing simply uses your Yahoo! e-mail account that goes with your profile and sends it to the e-mail address reserved for the group. So you are seeing the e-mail, but it is a public e-mail to the group's e-mail address (notice how the to: e-mail address is the same as the group name) and is supposed to be displayed like that.

So to sum it up this is the way it is supposed to work. It isn't something that is very obvious though; I don't know too many people who use Yahoo! groups very often except on some specialized topics.