I agree this would be useful, but I think it has to do with how reports work with editing and whatnot vs how they work on a dashboard.
If I want you to see data in a report, I can drop it on a dashboard without giving you access to the report or the underlying sheet(s), whereas if you need to edit that report, you have to have at least editor access on both the report and the underlying sheet(s).
Since I can set the dashboard widget to be visible based on MY access, I think it may have to do with confirming who is actually accessing the report by clicking on it.