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You can actually use Emirates miles on a number of partner airlines, including Alaska, JetBlue and Virgin America, but the number is really limited and you won't find a major US airline on the list. We would advise you to use Emirates miles on Emirates flights as the transfers are rarely 1:1 and there's a possibility that you'll lose some portion of rewards on transfers.
Airline rules are really strict sometimes, they try to tie you to themselves or partner airlines, cause it's not profitable for them to channel your money elsewhere.
There's a way around it, though. A travel credit card can let you earn miles that you can redeem on any airline of your choice. Such credit cards also make accruing miles much easier as they let you earn not only on flights, but on all other purchases.