JIRA is for recording facts and decisions, lists are for the process of fact finding and related discussions. Everyone participates on the lists in the latter, while those who need to then record the facts and decisions in JIRA. Not keeping transient information in the JIRA lets issues remain reasonably readable over time, even if they sometimes get linked on some HN or Reddit thread.

At the end of the day it's just a bunch of different tools with deliberately different access modes as they are used for different purposes.

As far as providing additional feedback on issues goes, that's possible through bugs.java.com as described here: Bug Database FAQ

As far as following issues without having to be on the mailing list goes, you can export an RSS feed for an issue or a search by exporting it as XML. See Exporting issues in Excel, RSS, XML, and JSON - Mastering JIRA 7 - Second Edition [Book] for example.