Up to 38% of Legit Mail Blocked as Spam

The latest Return Path email delivery study revealed that 17% of opt-in messages are erroneously blocked as spam by ISPs and email services. The number can be as high as 38% (Mail.com) and must be credited for a lot of the trouble with email lately. Yahoo! Mail's 4% look great in comparison.
Isn't it strange that it ever got accepted and common practice to block mail (and often not even send an error back)? Matt Blumber, Return Path CEO is quoted by MediaDailyNews saying that "ISPs and system administrators aggressively - and appropriately - try to protect their users from spam", but blocking mail is hardly ever appropriate (with the exception of hacker attacks maybe).
Installing SpamAssassin at the server so users can filter detected spam (which comes with a marker in the subject) easily, handing Bayesian spam filters to their users and teaching them how to complain about spam seems much more appropriate (and effective).