Don’t compare Harper with Bush or Obama, our system is totally different. While you have a valid point when you say that Bush and Obama don’t have unbridled power, the same cannot be said of Harper.
In USA, president can only propose legislation; it is up to Congress to pass it. There is no guarantee that president’s own party will support the legislation proposed by him.
In our system, the PM has considerable amount of powers, much more than a US president. He sets the policy, the agenda, and it pretty much is enacted into law. Even in the minority situation, most of what Harper proposed was enacted into law.
So if things go wrong here, it would be totally appropriate to blame Harper (or if things go right, he gets the credit).
So are you saying that Harper would have the power to prevent an MP from floor-walking, or that he could pass a law without support from beyond his party when in a minority situation? The only reason he has the power he has is precisely because his MPs choose not to floor-walk and because other parties support his policies.