Not that I agree with the justification, but I believe it is this: because Musk is involved in multiple companies, their choice is give him the amount he wants

Not that I agree with the justification, but I believe it is this: because Musk is involved in multiple companies, their choice is give him the amount he wants or accept that he'll be less motivated/interested in spending time on Tesla as opposed to the other companies, and that it would be a big enough negative to make this huge pay deal a better choice. I believe I read a couple of months back that this line of thinking even came out of Musk's mouth, but I'm not 100% sure if he said it or others did. Personally I think Tesla would be fine even if he quit completely, yet alone just spent less time on it, but if you believe that the difference between Musk caring about Tesla vs. not would lead to a difference of hundreds of billions of shareholder value, and you believe that without giving him then 50 billion dollars he will lose interest in Tesla, then it would be a logical business decision.