This is not price fixing. One must consider the two roles being played - the role of the manufacturer and the role of the retailer. The manufacturer wants the consumer to buy their product. (Coke wants you to buy Coke not Pepsi) They don't care where they buy it but rather they buy their product. The retailer wants the consumer to shop at their store. (Walmart wants you shop there and not at Target) They don't care what you buy there (Coke or Pepsi) but that you come to their store over some other store. This often puts these two parties - the manufacturer and the retailer - at odds when it comes to pricing because pricing is often used to entice consumers to buy a particular product or shop at a certain store.
Pricing is also used to position a product as prestige, economical etc. Pricing is part of the whole package is setting how the consumer perceives the product. When a retailer sets too low of a price, either by the pricing or coupon discount, sale etc, of a product that the manufacturer wants to present as high end it can also cause problems.
In a 1911 US Supreme court ruling (Dr. Miles Medical Company) mandatory minimum resale price agreements between a manufacturer and retailers were per se illegal. This interpretation of the law held until about 6 years ago when this issue was again brought up before the US Supreme Court (Leegin Creative Leather Products Inc. v. Kay’s Kloset). In this case the Court agreed with the argument that economic environment had changed since 1911 and that now manufacturers could set minimum price agreements but the "law of reason" would apply. The "law of reason" meant that manufacturers must consider all the circumstances surrounding the agreement to assure that it does not restrict competition or facilitate other pricing which is considered illegal.
So, in a nutshell, it is not price fixing and it is not illegal. Probably more information that you expected. Sorry, got carried away. Used to be a professor in business at a university. At least you got the short lecture!