This is very odd. I thought the "mark as spam" button was entirely within Gmail. I had no idea it sent a message to the email sender. Is this standard? It seems counter productive, since it confirms the spam was received. I want it to fail silently and disappear forever.

What should I be doing, to achieve silent spam disposal within Gmail?

I hope I don't have to set filters myself, that would defeat the entire purpose of a smart system that learns to recognise spam.

Edit: we're talking about "mark as spam", not "unsubscribe", right?

Yep.

Sounds like an easy way to verify active email addresses.

It is! But it's better just to check the SES queue for "email was delivered".

Yeah the SMTP protocol handles verification of mailbox existence already.