Apple continues to release security patches on older versions of OS X for a while. In my case, I am on Yosemite and have not upgraded to El Capitan yet, but I have all the latest security patches.
No I totally get it about the security updates; but it's the principle that I can't opt out if my own dumb ass wants to. Ultimately I would absolutely opt-in for security updates, but they don't seem to come stand-alone. If I'm not mistaken, Apple can push critical security updates these days anyway - there's been at least one.