And that brings up a question : how are posts not published through the proofofbrain.io interface treated?

I, personally, do not pay attention to the originating front-end -- if it's good content and either uses the #proofofbrain tag or originates on the proofofbrain.io front-end, I consider it; however, I believe @calumam has publicly stated that he does not vote for any posts that do not originate on the proofofbrain.io front-end. The rationale behind that is the authors who post using the POB front-end receive 80% less (40% of the total rewards instead of 50%), with the balance going to the @pob-fund (which is dedicated to maintaining the front-end). He is honoring their commitment to the tribe by only voting for content originating on the tribe's front-end.

All quality still receive the deserved rewards or there's a reduction?

There is no reduction in rewards per se, except that some folks who might have voted for your content (if it had originated on the tribe's front-end) probably won't even look at it.