The no-sourcing rule here is unusually firm and the rules page says why: every forum on this subject that allowed sourcing became a marketplace with a discussion section attached.
I believe it, having watched it happen twice. What I am interested in is the mechanism. It does not seem to be that moderators get lazy.
It is an incentive gradient, and it does not need anybody to behave badly.
Discussion is unpaid. Selling is paid. So the people with a commercial interest have a reason to post constantly and everybody else posts when they have something to say. Within a year the most prolific posters are the ones being paid to be prolific, without a single rule having been broken.
Then it compounds. Vendors arrive openly. Then vendors posing as members — which is much more effective and much harder to spot. Then threads whose real purpose is to move product but which are formally indistinguishable from a discussion thread. Then the genuinely knowledgeable people leave, because arguing with someone whose income depends on not being convinced is exhausting and nobody is paying them for it.
The evidence discussion does not get banned. It gets outcompeted for attention.
Which is also why the rule has to be absolute rather than case-by-case. A discretionary version requires a moderator to judge intent on every post, at scale, against people who are motivated to be ambiguous. That is unwinnable.
A flat rule is enforceable by somebody with ten minutes and no telepathy.
The corresponding cost, which the rules page is honest about, is that this forum cannot help you with the thing a large fraction of arrivals actually want help with. That is a real cost and it is being paid deliberately.