That makes it easy, within HN, to see if a company is just doing copy/paste spam, or if they're posting new/updated info each month. It also has the advantage of being easily verifiable (and enforceable? not sure what the enforcement actions would be...) here on HN versus random anecdotes of "I applied but never heard back", which I doubt would have enough weight for anyone to do anything about.
Users here could help police/moderate by simply replying with a link to last month's posting if there is one and the posting omitted it. That would somewhat-gently "call out" the company for not reading/following the rules, without users leaving negative comments on the thread.
(Just thoughts from a user skimming by, I'm not in the market for a new job at the moment so I have little skin in the game)