Automatically whitelist known mobile device providers?

I think I understand this switch in a historical sense. Once upon a time it was common to send direct to your SMTP server. Setup was limited and data costed alot, so whitelisting mobile providers meant that your phone or laptop (with its limited costly data) was not really a risk to you being spammed.

But then some telco's tried to be good netizens and blocked port 25 to anywhere other than their own SMTP servers. I would have thought that this option would be defunct at that point.

Since then, it's become far more common to use a secure method to send mail, and we also have autoconfiguring which *always* recommends you send via the CPanel server securely by default.

It's far more common to get infected mobile devices than it used to be also. That, combined with higher and cheaper data plans would make mobile devices a legitimate target for relaying spam.

So why does this option exist and why is it "on" by default? Is it purely a hangover from the past or is there another reason why mobile device providers are so secure that they should be allowed to bypass our usual checks?