I'm not sure you're receptive to this, but I will tell you what would have happened had Pittsburgh been called for "assisting the runner."
The touchdown would have been nullified, the Steelers would have taken a penalty, and then they'd have gotten another shot at a 3rd down touchdown. A penalty was not called, which doesn't surprise me because I've never seen it called in all of the many hundreds of NFL games I've witnessed, and so the touchdown counted. Except that then upon review the touchdown was nullified, and the Steelers had to settle for 3.
So the Cards got a break.
The fact is that many calls could have gone both ways, and the Cards got A LOT of breaks too. The holding call in the end zone, the overturning of Warner's first fumble, the fact that Warner took off his helmet to argue that call and wasn't flagged, the 80 non-calls on Gandy holding Harrison, Taylor's unsportsmanlike conduct on Boldin, etc. You just don't hear as much complaining about those calls because the Steelers won and because, well, their fans aren't a bunch of babies. I'm not referring here to Cardinals fans. All of the whining I hear is from Cowboys, Ravens and Titans fans who are upset about the shortcomings of their own teams.