Because if he didn't think the government would approve the deal, he shouldn't have made the offer in the first place. By making the offer he is creating a massive amount of distraction and uncertainty for the company-to-be-acquired, and if in the end the deal is voided because the government blocks it, then the company-to-be-acquired need to be compensated for all the wasted time and effort that went into a failed deal.

This is very common in large acquisitions and mergers. Remember, Must didn't have to include the $1B walk-away fee. But if he didn't, then the board may have been much less likely to accept the offer.