let's start with - legitimate = conforming to the law -
Kraken is out of law because it's not a licensed money transmitter business(MSB) in any state so they are operating illegally (in USA for sure)
What does mean? Well, we know what happened with other unlicensed exchangers in the past . Overnight, FBI closed their websites and they arrested the owners. Client's funds? Gone !
So, you decide if you assume this risk along with others that derives from BTC market.
Most of Kraken's customers are in Europe, and they dominate the BTC/EUR trade. I don't think they have many American customers at all
They have and they ACCEPT.
By accepting US clients, you, as company must be registered as MSB in the States where you want to operate. More than that, Kraken is incorporated in USA so, it doesn't matter whether they have or not clients USA.
It's an act of craziness to work from and with US clients and not having a proper license for that. You can be closed anytime and the Feds can put you in jail without any issue. We have many cases with unlicensed exchangers who are behind bars because of that.