there are only 4 licensed exchangers in USA. USA is well known as a country who enforce the financial laws pretty well.
Licensed exchangers in USA : Coinbase, Circle, Itbit(for New York only) and Gemini(NY only) . The rest of the US exchangers are OUTLAW (XAPO is one of them) and they can be shut down anytime as it already happened to many...
Xapo is not only a...wallet. You can buy BTC from them, they have debit cards as well. That means it is required a money transmitter license.
Example:
http://www.coindesk.com/bitcoin-firm-xapo-cuts-north-carolina-accessAlso, Xapo is lying like a shit
"Xapo has officially relocated its corporate headquarters to Zurich, Switzerland," (May 2015) while on their website is saying:
"HEADQUARTERS: Xapo is incorporated in Hong Kong , has an office in Palo Alto, California and operates Worldwide" and MORE in their TERMS "
https://xapo.com/terms/" :
"If you are using our Services in the United States, you are engaging not with Xapo Limited but with Xapo, Inc., a Delaware corporation" 
However, they are not licensed ANYWHERE.
Really? most people here on the forum use and share that Xapo is one of the best of the best ?