Presumably because third party insertion doesn't give the app creators sufficient control over what's shown. This happens all the time. Forbes.com starts servin

Presumably because third party insertion doesn't give the app creators sufficient control over what's shown. This happens all the time. Forbes.com starts serving malware and then cry "we didn't know! the third party guys just put whatever in that box!" So it's not (just) the outward flow of information, but the additional policing burden of a separate inward flow of graphics.