Catcher hits bat with throwback

[Tim C]

I have a question:

We know if a runner is stealing third and the catcher's throw hits the bat of the hitter in the box (who has done nothing intentional) that is "tough nuggies" as the batter does not have to disappear,

So if F2 throws a ball back to F1 how would we know if it was actually just a "toss back" or if F2 was trying to pick R2 off,

So my question is:

Why is there an apparent difference in a "play " by F2 or simply a "return throw" by F2 and the resulting play is handled differently?