Data transfer costs. We host some local mirrors for some software distribution, and (inbound) data transfer is the biggest downside. That said, once the data is up, outbound data down here is un-metered so it has a much bigger advantage.
Well, strictly speaking you get charged for outbound data down here as well but only when you look at it realistically. i.e. there's a limit to a hosting plan for $x (anybody offering unlimited needs their head read).
But yes, generally data transfer is measured in both directions in the US.