I stumbled on this issue for another Disqus problem I have, but I too have noticed problems with this.
It seems that the the Disqus plugin only looks at the global setting. If you have comments turned off as default, it is not possible to turn them on on individual posts.
Even if you first turn them on for the post, then also globally Disqus will show up for already existing posts. After turning it on globally Disqus simply will only show up for new posts.
Pages does not have comments turned on by default, still Disqus shows up and there is nothing I can do in the UI to fix this.
The only solution in that case I have found is to hack the code as described on this page http://zackeryfretty.com/removing-disqus-comments-from-wordpress-pages/