Having a tipline isn't so bad on its own, but then the problem becomes: User leaves unverifiable tip, what do you do with it? It's not like you can break the encryption...
It already is one for all intents purposes, just not one that is used ubiquitously (except in the sense of getting us to buy stuff). All that is changing is that the panopticon is becoming slightly more explicit than implicit, making it that much easier to (eventually) flip a policy switch that designates groups wholesale as "enemies of the people" requiring active scrutiny and interference.
I'm reminded of this cartoon:
http://2.bp.blogspot.com/-x2BP9Xd7xfo/T7bvuweLPfI/AAAAAAAACj...
Twenty years later and it is still fresh as a daisy.
Bonus clip: https://www.youtube.com/watch?v=UQ6LGrr8iEg
The downside is, context matters. If you report messages out-of-order or with important context deleted, you can trick investigators into thinking something was said or implied that actually wasn't. It needs to be carefully designed.
(I blog about cryptography, but you should ask a cryptographer if you want to design something like this.)