Consider this situation from the perspective of a biological person. A biological person has resource limitations in terms of effort, concentration and resources. When you're already worth say 10bn each, your primary limitation is time and effort, which doesn't scale. Do you really want to squander your personal time and effort on increasing your net worth by 5%? Not likely. You have the next horizon to conquer and you have plenty of cash.

Let Zuckerberg have his measly $750m, which means F all to either of you and get on with your next project.

This is entirely a pragmatic and reasonable decision on the part of the whatsapp founders.

It's not obvious that they would have won. Zuckerberg hasn't actually forced ads into WhatsApp yet, and WhatsApp remains ad-free today. He just (allegedly, according to anonymous sources) is suggesting that WhatsApp leadership should do it.

Maybe the 'bergs promised some sort of horrible countersuit, aimed at clawing back the billions? Some sort of breach of contract or duties to shareholders or something? Even if the claims were obvious bullshit, why take a 30% chance of losing one's ginormous fortune? In fact the whole thing could have been a way of extracting some further, more-recently-dated agreement that FB and the WA founders are square...