The example site provided ( https://greggman.github.io/doodles/html5-webcam-iframe.html ) serves an iframe from a different domain so the site settings on the visible page do not affect the actual camera permissions:

Site settings: http://imgur.com/a/JIDeD (note: camera is set to 'block', but there is still the red dot in tab indicating camera access)

Camera usage: http://imgur.com/a/3HXHo (note: different domain listed)