Any manufacturer can put whatever they want in their reseller or distributor contracts. If a business wants to be a reseller or distributor of a product, they can decide whether they are willing to accept the terms of the reseller or distributor contract. And they can also decide if they want to abide by it after they've signed it.
So, if Fender or Apple or whoever wants to specify that a product cannot be sold for less than MSRP, they can absolutely specify that in the contract that BestBuy or Rocket Music or whoever has to sign in order to become an "authorized reseller". Then, if BB or RM does sign on as a reseller, they can decide after that whether to actually stick to MSRP or not. If they don't, then, one, you can argue that they are being unethical, and, two, it is obvious that they are in breach of their reseller contract and, if Fender or Apple or whoever finds out, they can pursue whatever remedies are specified in the reseller contract. And that could be anything from simple termination of the contract to punitive damages.
As a manufacturer (of software) myself, I can totally understand why a manufacturer would put into their reseller contract a clause that specifies a Minimum Advertised Price. If Rocket Music starts advertising basses at $10 over dealer cost, then all of a sudden there's a ton of pressure on Best Buy, Guitar Center and every other retailer of that bass to sell them at the same price. At which point, some or all of them will decide that they can no longer make money selling those basses and so they'll stop carrying them. Which is bad for the manufacturer.
Besides that, it's to Best Buy or Guitar Center's benefit to advertise them at a higher price than they'll actually sell them for because, as has already been noted, some people will just go online and buy stuff at the listed price. And the cool cats who call and get a discount because they are "in the know" will often also be the same cool cats that talk about the great deal they got from MF or whoever and, thus, influence others who may not be so "in the know" to buy from MF, and those people end up buying at the listed price because Cool Cat said that MF gives good deals so whatever the listed price is must be a good deal.