Actually, I'm close to thinking this might be the practical answer. It goes with the objection that Dave and others have raised: why is it that other list services apparently do not do this?
Groups.io cares on behalf of the other members using the same email service. The implementation on Groups.io started with the observation that a lot of group messages were not being delivered to certain services. And I think that means "bounced" and/or greylisted, not merely diverted to Spam. So Mark sought a way to improve overall deliverability to those services.
Perhaps the other list services take a less altruistic view of it. If you (a member) are fool enough to continue using a service that doesn't deliver the mail you want to your Inbox, well that's your problem. The fact that your problem is (in part) caused by the actions or inattention of other users of your email service doesn't mean that it isn't still your problem.
But... (there's always a but)
Would that choice merely trade one source of ongoing complaint and confusion for another, potentially larger one?
That is, "why aren't I getting the group's messages?" has long been a FAQ (at least in Yahoo Groups). People are told to check their spam folders and to add addresses to their filters and/or address books. And of course if the delivery through that service deteriorates to the point of routinely rejecting messages from Groups.io, that doesn't reflect well on Groups.io as a service either.