Atlas V currently relies on Russian engines. Atlas V is not "the US."
While it and Delta IV currently launch NASA and Air Force EELV (evolved expendable launch vehicle) payloads, that is changing with the opening up of NASA and EELV launches to New Space operators like SpaceX and Orbital Sciences. More will follow.
Delta IV, Delta IV Heavy, Falcon 9 v1.1, Falcon Heavy, SLS and the coming Blue Origin medium-heavy lifter do not use Russian engines. Atlas V could also evolve to use domestic engines.
Falcon 9 v1.1 is one launch away from qualifying for EELV launches and is already launching NASA payloads. Falcon Heavy will be able to launch 2+ EELV payloads at once, and 12.2 metric tons (2-3 communications satellites) to Mars.
Orbital Science's Antares depends on the AJ26, a derivative of the old Russian NK-33, but it will be domestically produced. They may even end up buying engines from another New Space outfit. They are already launching NASA ISS resupply flights, and with upgrades to Antares could also do EELV.
Again, Russia has mote to lose by such an action than anyone. Much more.
Exactly - Atlas V is commercial, not military.
This will seriously whack Russia right in the currency-exchange; if Russia is serious about hardening the ruble (Russia's currency) you don't reject exports to nations that actually HAVE harder currency. (Russia may not like that the United States remains the hardest currency issued by any government - while their own ruble is one of the softest - but that is economic reality, and it is primarily because the Russian marketplace is hostile to imports from anywhere; the same has been true of not only the PRC, but even Japan and most of the "Asian tigers".)
Russia also has to take India seriously - unlike the PRC, India IS actively looking to commercialize their space venture - do they really want India to take the rest of Russia's lunch?