i don't think "legalize" is the correct word to use here because it is somewhat misleading. they just regulated bitcoin mining because it was not illegal before to legalize now and if i am not mistaken, legalize is a term used for when the government turns something from illegal to legal.
I imagine the US government will take this as cause to further clamp down on exchanges and other Bitcoin services. They'll push for stricter regulations and enforcement under the guise of concerns over sanctions.
if they do it they will only use it as an excuse but the real reasons would be different. these sanctions didn't start yesterday, they have been there for years and no Iranian could ever use any of the bitcoin exchanges since none of them ever accepted them anyways!