I mean, sure, it breaks lists. But the lists were already broken, and yahoo is just saying "we want it done right." It would be unfortunate if they lose people because of doing the right thing, technically.
It's only "the right thing" for point-to-point email. No mailing-lists, no email forwarding, etc...
Is it so hard to change from to the address of the mailing list (which the email REALLY IS FROM, and set the reply-to: to the original author? That sounds like the right thing, to me.
A very common use of a mailing-list is an an SMTP implementation of a forum. Last I checked, forums don't send messages, only their members do. So no, the message is really not coming from the mailing-list.
So yes, it's easy to do, but it's wrong.
Stefan