You might be right. The dashboard as far as I can remember has never used ACL's it just uses "my" or "all". The feature where ACL support for case was introduced might have been for a different need and doesn't seem to have included the dashboard (https://issues.civicrm.org/jira/browse/CRM-17645).
So yes and no to your question?
It was done deliberately in 2010 and it's been that way ever since. While it's not stated in the JIRA link below, if I had to guess the reason it's because it might allow you to give your buddy access to a case they shouldn't see.
Try the case summary report, which might not be visible by default. Go to Administer - CiviReport - Create New Report From Template. Then pick the Case Summary report. Then on the filters tab set your filters.
You USED to be able to do this but the system was changed a little over a year ago so that it now removes the original from the non-case side - see Items not showing up on activity log.
There is the copy-to-case action for most activities on a case but it's not available for Emails/Inbound Emails. The reason might be that historically the "spirit" of ...