As I said in that thread, I think this norm is unhelpful for encouraging adoption and building a diverse community and information flow.
Authors/publishers, both in the large and in the small, write and post where there is engagement. If there is no engagement, there is little incentive to post. But also, if there is nothing posted, there is nothing to engage with. Chicken, egg. Vicious or virtuous circle.
Content mirroring provides a way to test the waters, so to speak. It provides a zero-marginal-effort means to get your content onto ADN and see if the community there is interested in it. If there’s engagement, (A) reciprocate the engagement, and (B) consider making dedicated or channel-specific content. This does require that you read ADN, monitor the mentions, etc., but none of that is visible in the “posted with IFTTT” blurb.
Given the existence of the low-cost means of testing viability of ADN engagement, it makes little sense not to use it to test responsiveness. The norm of “mirroring not welcome”, however, is like saying “if you want to swim, dive; put your toe in the water and we’ll shoot.” It doesn’t allow people to enter gently, and then have the opportunity to follow up and commit further.
Finally, assuming that those who have paid $50 for the service are not interested in engagement based only on how their stream is populated is extremely uncharitable. First, how they post implies little about how they read. Second, perhaps they would like to engage, but haven’t yet found community; continuing to post, regardless of method, may enable community to find them (e.g. via the global stream). And sure, there are probably some who are indifferent, but that will happen in any sufficiently large community and I’d wager that the $50 entry fee means it happens less on ADN than elsewhere.
If you don’t want to follow people who mirror their content from Twitter, fine. Don’t follow them. Mute them if you want to. Better yet, try engaging with them and encouraging them to step in deeper.
But please, attempting to make that preference a broad social norm with shame for violation is poisonous. It says that people, publishers, etc. are only welcome if they are fully committed from day one. This creates a toxic environment for those who would engage, but are not sure if they or their ideas are welcome or will generate response. The answer they are getting is a resounding no.
P.S. Cross-posting well does require care. Cross-posting @-replies, or even mentions, is rather noisy and confusing, particularly due to username mismatches. But other content? I don’t have a problem with it, though I don’t automatically cross-post myself (I’m more likely to do ADN → Twitter, or use a third service that posts to both).