People think of these issues too much in terms of black and white.

You should think of it as a negotiation. Many transactions, even on a smaller scale, are complex, and deciding on the "true value" is often impossible. So the tax authorities are negotiating with Facebook, and Facebook's report is simply its opening offer. You wouldn't consider a salesmen who overpriced his product to be doing something necessarily unethical (assuming they weren't outright committing fraud or otherwise doing something illegal).

Disclaimer: Not a lawyer or accountant, and I'm totally generalizing here from other transactions I know more about. I actually know almost nothing specific about the US tax system or this situation with Facebook, I'm trying to convey the general attitude that most companies have.