It's basically a good faith exception. At the time it wasn't known it was illegal so the police couldn't have known. Now that they know anything forwarded becomes inadmissible.
Agreed, there are plenty of mail services that are geared for just sending only on your behalf. They'll make sure your mail is secure and actually gets to the recipient's mailbox instead of getting booted as untrusted spam.
It's basically a good faith exception. At the time it wasn't known it was illegal so the police couldn't have known. Now that they know anything forwarded becomes inadmissible.