This is not quite right. The taxes on a reward are based on the program from which the miles are withdrawn, not from the operating carrier. So a CO reward on AF metal would only pay the CO-required fees (basically federal taxes) while an AF reward on CO metal would be subject to the AF-required fees, including the fuel surcharge. The worst part about that is that AF would just pocket the surcharge as pure profit since CO would never even ask for the money.

And if in coach, flying on an international carrier has a much better chance of seeing free booze served, if that is important to you.