Last I knew it is supported, but US account holders can't make payment to account holders in other countries. They basically split the system in half to comply with the US's anti online gaming regulations. It may have changed in the past few years but this was the case last I had hear.
Yes it is working that way US customers can send but not receive money from other countries and also can transfer domestically
Maybe that's the reason for low usage of Skrill by people over here compared to Paypal. And later one is going down also with ebay spinning off the Paypal.
yes they are available in USA but paypal is more known because of their marketing. Nothing else. They eat off many people money without reason and still they are giants. So, if you want some clients paying you direct, ask them for payoneer. they can pay you using credit card at payoneer after being signup there.