- It's not as if the support period for 13.04 was sprung onto people unexpectedly - it was set out in advance of release.
- And it's not as if there's no alternative for those needing a longer support period - there's always an LTS or two to choose from.
People who choose a short-support-term version and then don't update ought to know the potential downfalls of that predicament already and be taking their own responsibility to watch security announcements. If they are not doing this, the responsibility to educate them does not fall to a site like AskUbuntu. It's not that these people are beyond help, but that it doesn't make sense for this site to be the ones reaching out to help them. AskUbuntu is not a site for disseminating public notices about security issues - it's for answering people's questions about Ubuntu.
The site has an established and quite reasonable policy to consider questions specific to an older, unsupported version of Ubuntu as "off topic". This is a sensible and appropriate policy, and it does not just serve to keep content relevant but also to reinforce the idea that these versions of Ubuntu truly are "unsupported" - no matter what the question, the answer to a problem in an unsupported version is basically "have you considered upgrading?".
To break this policy in order to spread the word about one particular security flaw would be inconsistent and set up an ambiguous precedent, sending a mixed message where it's OK to help people out with their unsupported installs if the issue is "important".
So, what is an alternative solution?
If a question (whether it is about Heartbleed, or anything else) is specific to 13.04 then it should be closed with a message directing people to a big "why you should not be using this version and how you should upgrade" wiki page. If we want to help people get off vulnerable versions, that wiki page can be their lifeline and the limit to the support we want to give them.
No cherry-picking which issues are or are not "important" enough to break our policy on supporting retired versions of Ubuntu.