Generally, if you have not actually assembled something illegally, AND you have a means of using it LEGALLY (like, in this example, having the pistol upper on a pistol lower or SBR lower), then you're fine. Where you get into trouble is buying the pistol upper WITHOUT a pistol or SBR lower.
Note that the AW laws make it almost impossible to make a legal post-ban AR-based pistol, because one of the restrictions is that the gun must weigh no more than 50oz. Post-ban AR pistols also cannot have flash suppressors, threaded muzzles, or any kind of forward grip or heatshields (so the handguards have to go). You can get around this by permanently affixing a magazine to the lower, but otherwise you'd need to customize a lot of parts.
Those uppers are really only useful for folks with preban AR pistols (rare), or who have machineguns or SBRs.
And as Chuck pointed out, they are equivalent to a .22LR in terminal ballistics, so as a weapon, they are pretty worthless.
-Troy