Subfolders' event portlet isn't contextualized, whereby the search base becomes the contexted or current folder automatically.
For example, on a Plone site, there's a committees page, with an event portlet that shows the top 5 upcoming events from all committee page and each committee page has an event portlet with their own events.
With the current implementation, the site manager has to set the search base for each committee manually, instead of automatically inheriting the event portlet with an option or value to set the search base to the current folder or context.
For more information see issue 276 on plone.app.event