Even though it's self moderated, they should know that deleting a complaint is a bad PR for them. It's more honorable when they attend to such complaints there, it shows responsiveness and accountability on their path. They may want to delete spams or off-topic posts on their thread, same reason they make it self moderated
The original purpose of self moderation of thread was so the OP can filter comments that ain't meaningfully contributory to the thread such as off topics comments and spams but in some cases of deleted comments in self moderated threads it's more of a personal decision of the OP, some borne out of sentimental to views which doesn't align with OP's.
Yet, the notion of self moderation makes them. Similarly, some service threads that are said to be self moderated can choose to filter away certain comments that provides information that doesn't favour their service, hiding under the idea of self moderated thread.
Perhaps, for service providers as recommended by OP of this thread, they should report any comments that seems to them as spam or off topic to forum moderators to be deleted, it's much better than way as it would help prevent scammers from taking advantage of the self moderated feature to cover their quick revelation.