I have no problem with Bell slowing down my P2P transfers when there's congestion. However, from experience, Bell throttles based on time and not bandwidth/congestion.
As soon as the throttling times are over, the P2P downloads speed up considerably, so it's obviously not throttled based on congestion on the network.
As for downloading when it's not affecting others. It would only affect you if they were experiencing congestion, not based on time of day; which is what they have implemented.

Bell is artificially slowing down their customers during certain hours of the day because they are unable to implement a correct solution to handle congestion.

wrt airplane fuel:
Actually there are some companies out there that do payload and fuel optimization with knowledge of fuel prices, payload weights, aircraft performance, forecast wind patterns.
There is also a cost in flying with more fuel than you need, as it takes fuel to transport fuel to your destination.
The algorithms for figuring out how much fuel to put on board aren't particularly difficult (at least when I was working on it) it's just _really_ computationally expensive, and also estimations upon estimations, upon estimations, upon predictions.
Another thing to be considered: the way the plane is packed will also adjust the amount of fuel they use. So when you're packing a plane, you've got to deal with weights side to side, and front to back too.