Actually it's a fairly recent invention. It only seems "standard" if one has no recollection of a time and a place (not so far back) when outsourcing (and the systematic discounting of blue collar workers, generally) wasn't nearly as prevalent as it is today.

They may be working at Facebook, but they work for their own company.

We all acknowledge that that's the way things (technically) are. What's at issue is whether that's the way they ought to be.

And the extent to which, while it may be technically correct that these people are working for different companies... on a deeper, more fundamental level -- it's just a big gimmick.