Lots of confusion parading as truth on this thread, in part because people are mixing MSRP and MAP. Two different things.
Manufacturers Suggested Retail Price is the price the manufacturer recommends that a product sell at. Traditionally, in photo equipment there has been wide variation between MSRP and street price (actual price generally paid). The difference has narrowed in recent years. Purpose of MSRP is to give retailers an idea of what the product should be selling for, according to the manufacturer. Depending on the manufacturer this could be accurate or a total fiction. Sigma, for example, tends to set an unrealistic MSRP on their lenses, that almost no retailer honors.
MSRP is just as the title says: "suggested." My understanding is that in the U.S. at least, there are legal restrictions that prevent manufacturers from enforcing the MSRP as a firm rule. Thus, the growth of MAP or Minimum Advertised Price. MAP says the retailer cannot advertise a price below a certain number.
MAP is designed to help smaller retailers compete. I don't know the specifics of either Canon's or Nikon's dealer agreements, but it appears they enforce the MAP during rebate times (which have become almost year round). No retailer can advertise an item for sell at less than the MAP and still participate in the rebate program.
The issue, of course, is what constitutes an "advertised price." Again, not knowing the specifics of the agreement between the manufacturer and the retailer, I can't say for certain. But, it appears that there are some exceptions. As the OP suggested, it appears that MAP does not, in some cases, apply to auction sites. Similarly, it appears that there are exceptions for direct links from websites. For example, both Canon Rumors and Canon Price Watch have occasionally offered special links for discounted prices from either B&H or Adorama. In those cases, if you follow the website link, you receive a lower price than if you go directly to the merchant's web page.
I don't know if these special deals and auctions violate the MAP agreement, but I suspect that the big merchants like B&H and Adorama wouldn't use them if the did – too much at risk.
Is MAP price-fixing? I doubt it. Again, not knowing the specifics of the agreement, makes it hard to know. But, from what I understand, MAP is linked to a benefit given the retailer. For example, if you want to participate in the rebate program, you have to adhere to MAP. The retailer has the choice, they could turn down the rebate program (of course no one would do that), so it's voluntary and not mandatory. Similarly, most manufacturers offer "co-op" programs for advertisements (If you advertise their product, using their suggested marketing materials, they compensate you in some way) Again, it is not coercive, but it is a strong incentive to adhere to the agreement.
The voluntary nature of the agreement probably keeps it legal. If there would be a successful legal challenge, I suspect it would require proving that the agreement is not truly voluntary, but is so coercive that it is in fact price-fixing. However, one thing the manufacturers have going for them is that they are clearly in a competitive market. Canon is not colluding with Nikon or Sony to fix prices, they are simply using incentives to keep their prices consistent among retailers.
The above is based on my understanding of typical retail practices and isn't meant to imply I have first hand knowledge of the specifics of the agreements between Canon and its retailers. Nor do I pretend to be a trade attorney (although I did stay at a Holiday Inn recently).