I just took a look at a few lists I’m on. Of about ~ 15 google groups, all of them have reply-to-list as their default (not sure that reply-to-sender is an option). Of about ~ 15 GNU/Mailman lists, all of them had reply-to-individual as default. I don’t have enough yahoo group subscriptions to make any statements about them.
So perhaps it’s a philosophical choice that the different list providers make on behalf of list admins?