People are overlooking the fact that a poison pill will likely increase the price of Musk's final offer, and in doing so will maximize shareholder value.
Put another way, Musk may be willing to pay a lot more than a 25% premium for Twitter (he already said he doesn't care about price). Without this poison pill, Musk can force a takeover by accumulating shares. With this poison pill, the board has leverage to maximize his final offer.