The thinking behind the ACL preprint policy

In October 2017, after much discussion and surveying, the Association for Computational Linguistics (ACL) adopted new policies on submission, review, and citation. A key change was to disallow conference submission of papers that have been posted or updated as non-anonymous preprints during a period from one month before the submission deadline until after results are announced. I have seen several online discussions mainly critical of this preprint policy, variously describing it with terms like "idiotic" and "ineffective". I think the ACL has failed to sufficiently explain to people the thinking and discussion underlying this aspect of the policy, so here is my attempt to do so. Speeding up the rate of scientific progress by fast dissemination of results is a good thing.