He would owe back taxes on the coins he mined

It's not that easy because no one recognizes his face or anything, and there have been many people or anyone claiming to be Satoshi Nakamoto the creator of Bitcoin, but that's just a claim.
And if one day Satoshi Nakamoto does appear, I don't think most people will believe it, and assume he or they are not the real Satoshi Nakamoto because there is no accurate evidence.
Satoshi Nakamoto will remain a mystery forever, and if US authorities had the ability to pull it off, they would have done it a long time ago.

No, the US authorities wont put him in prison because it's made for the criminals only and Satoshi isn't a criminal but a hero who has given us the gift of Bitcoin. Many people have claimed to be Satoshi and none of them is in prison and even if real Satoshi claims to be Satoshi then still people will reject that claim. A genius like Satoshi knows very well that if he reveals his true identity then his 1 Million Bitcoin would be in trouble, and he would never do anything that could cause him financial harm.

The Satoshi deserves a Nobel Price in economics for the asset he has given to the world. He's the one who opened our eyes in this technological age. It's because of Satoshi we understood that we can have a currency which isn't controlled by governments. He's the mastermind who has shown us the power of decentralization in the form of Bitcoin and such a person deserves a Nobel Price for his innovation. I believe that Satoshi is also one of the most influential people of this century and they best thing about him is that nobody has seen or listened to the voice of this amazing guy.

We already have too many Faketoshis and it will be difficult for anyone now to prove that he is real satoshi. There are few ways one can prove he is actual satoshi, like moving bitcoin from sleeping wallets of Satoshi.

One thing is very clear that government and other agencies won't spare him and will definitely come after him. Its better for satoshi not to disclose his real ID at the moment.

On what grounds would he/they be arrested ? Seeing that no founders of other alternative coins have been arrested for same reason then Satoshi has nothing on him/them to be put in jail.  But why the US?
I don't think Satoshi would be bothered about winning a Nobel Prize.
Sotoshi's anonymity is fine that way, for what is worth, bitcoin is doing great and I think satoshi's anonymity could just be part of the reason why the currency BTC is standing tall.

More likely Satoshi will be given a Nobel Prize in Economics.  He will be so popular that he might be more popular than any celebrities.  While, I do not think that there is a reason for the US authorities to put him in prison, if ever he might get assassinated even before the public knows who Satoshi is.  Since his creation is a threat to centralized financial authority.