It's safer as well. Some (most?) of those "unsubscribe" links have a unique ID hooked in with your email in some database so they know that they have a hit and can send more email to you.
Well, they need something to be able to tell which email to remove from their DB too.
Not all is as sinister as it looks.
The simply solution is to just can any senders that don't respond to unsubscribe requests. But those will be just as bad in plaintext as in HTML, so I don't see why Plaintext means it's not spam, they can do the same thing Plaintext.
