why does plugin-types inherit to nested browsing contexts?

While investigating a few CSP bugs in Chrome, I noticed this text in the
CSP 1.1 spec for plugin-types:

"Whenever the user agent creates a plugin document in a browsing context
nested in the protected resource, if the user agent is enforcing any
plugin-typesdirectives for the protected resource, the user agent must
enforce those plugin-types directives on the plugin document as well."
(http://www.w3.org/TR/2014/WD-CSP11-20140211/#plugin-types)

Dev (cc'ed) and I found this behavior a little odd and were wondering why
plugin-types is inherited. Is the goal to give a developer a way to say
"don't allow Flash to appear anywhere in the content area of my page?" Why
is this directive inherited but not any others?

Thanks,
Emily