Mavericks is free.
Its system requirements are the same as for Mountain Lion
http://www.apple.com/osx/specs/
http://support.apple.com/kb/HT5444
It is also a security update:
http://support.apple.com/kb/HT1222
http://support.apple.com/kb/HT6011
I surmise that Apple does not intend to provide security updates for older OS versions. At least not for 10.8.
If you want to get patched, start upgrading to 10.9…
Update: 10/4/2013
While it was fun to speculate about Apple forcing us to upgrade to 10.9. It was just speculation. I now believe I was wrong and I expect Apple to release updates for older versions similar to its past behavior.