It would be price fixing if Nikon, Canon and Sony agreed to sell at certain agreed prices. It would be price fixing if all retailers agreed to set a fixed price.
This is not price fixing. A manufacture can require it's retailers to charge a certain price. It happens all the time.
Actually according to US anti-trust law, manufacturer mandated prices to their dealers perfectly fit the descriptions of prohibited actions described in the Sherman Act, the Federal Trade Commission Act and the Clayton Act.
However...the acts also have language about "reasonableness" of the actions that fit the descriptions. Which in legalese means its up to the courts to determine who gets to violate the acts and who doesn't.
Obviously a manufacturer of some sort with a dealer network, was in the past, given a favorable ruling in a court case and now "dealer network price fixing" activity is essentially exempt from the acts as a result.
But that doesn't mean that dealer network price fixing doesn't fit the descriptions of prohibited activities under the acts, it just means the courts have issued an exemption is all...which of course if you're Canon, that's all you need and care about.