That's not the case. The ICANN is an entity independent from the US. Well, that's not entirely true, but true enough that the US can't just take a .com from someone just like that.
Frankly, I would prefer if the USA just used .us (and .com.us, .org.us, .gov.us, etc) as a ccTDL and left .com, .net, .org, etc to be under the supervision of the UN.