Its a quirk of the (immiegration) system
To give you some perspective. When you enter the US on a non-immigrant work visa -- say an H-1 -- then you're not immigrating but you are allowed to change your mind and apply for a change in status to 'immigrant' -- that is, apply for a Green Card. This involves a lot of messing about, including the whole Labor Certification bit to show that you'd advertised your job and nobody suitably qualified applied for it. The immigration system is perennially out of date so it assumes that if you're hired for Role 'A' then you'll be doing Role 'A' for the foreseeable future. In real life companies tend to move people around a lot from roles that are going away to new functions. They're all technically a new job and so strictly speaking they'd need to file Labor Certification and all that. Its a lot of messing about so they probably ducked it. I don't blame them (and I'm definitely not a Facebook fanboy, I've just been through the system so seen it from the inside).
The real motivation behind this is the salary. There are people out there who want that salary. They need it. Its their right. They'll do anything to get it. Even though it sounds a bit excessive for a generic job, its the sort of thing where you've hired in some serious talent and want to retain it but there's a bunch of "I got my CS and a couple of years Python/JS/Whatever, where's my six figure salary?" types out there who will moan, snitch and generally carry on until they've bullied someone into giving them what they feel they deserve.
(Oh, BTW -- if you become unemployed during your Green Card application period, between filing and getting your interview, your application folds and you're SoL. Back you go to wherever you came from.)