Anyone who has spent time developing for the web has heard (and hopefully taken to heart) the phrase "don't break the back button". The back button is one of the fundamental mechanics of the browser and the browsing experience and breaking its functionality is a fast way to irritate users.
By the same token, one of the fundamental mechanics of email is "Reply". I regularly receive mass emails from organizations informing me that I can't reply to the address that just sent me an email. Replying isn't just a fundamental of email, but a fundamental of communication. Imagine answering a phone call and being told that you have to dial a different number to speak into the line.
Using a do-not-reply address to communicate isn't just a practical annoyance - it sends a message that the organization's time is more valuable than yours. The organization has no problem blasting its message out to you, but if you want to answer you have to spend your time searching the web or scouring the email for a valid way to communicate.
I understand that a large email blast will generate all kinds of auto-responses, email bounce messages, and other forms of invalid replies, but with some decent filtering and a little work an organization could surely separate the wheat from the chaff. It's the least they could do to in exchange for reading their message.