The Reason why that black cubes cannot be on the top row with the purple arrow cube is - because of the rule that says cubes have to be stacked from the ground level and up.
If those black cubes was truly on the top row -- that would mean that the cubes just below that would be on the second row. This is impossible, because there would be no cube below that 2nd row supporting it from the ground level -- they would be *hovering* while being attached to the 2nd row of the purple cube tower, which violates the DAT PAT rule.
This is why the purple arrow cube MUST be part of the back row.
I hope this makes sense!
*EDITED / FIXED*