You are appealing to a concept of justice. Zuckerberg sees it a different way: how little can he get away with paying? If he can make a profit paying you $150K, he can make even more paying you $90K. He'll take it if he can. Whether he can depends on whether you have competing offers. If the economy tanks, maybe you won't.

If all their employees work from home, that's going to tend to equalize the salary levels across regions, and the number they settle to likely isn't going to be the SF Bay Area number.