TFA said that Facebook paid it his parental leave. If FB thinks it is worth it to retain him as an employee, it's their money.

The US government on the other hand seriously doesn't give a fuck about families since most of their programs are oriented to single mothers. Also there is no federally mandated parental leave http://en.wikipedia.org/wiki/Parental_leave

Why not, though? Assuming a democratic government represents the interests of it's voters, apparently a significant fraction (ok, at least one) of the population would be interested in paid 4 month vacations.