Sometimes street dates are broken by low level employees in the warehouses who don't know any better and are just trying to do what they think is a good job (get product in and out of the warehouse as fast as possible). I have some sympathy for the company if that happens.
However, really what we are talking about here is what either is or should be a binding legal contract. I have no sympathy for the deliberate decision to break a contract. I'm very much a liberal capitalist in outlook, but Adam Smith and the rest of the liberal capitalists pretty much agree that the capitalist system only works if it most sellers are honest. If the participants in a capitalist system - however motivated - don't practice good business ethics, then the basic assumptions of transparency, profit maximized by capital producing behavior, and so forth get tossed out the window and the fundamental model of the market breaks down. When that happens, you pretty much have to have laws to push the market back into its assumed model.
If sellers of books have a reasonable belief that thier profits depend on a particular release date and cause thier distributers to agree to a particular release date, then it is unethical to break that agreement and the sellers of books ought to punish the behavior. There ought to be fines. There ought to be some threat of giving exclusive distribution rights to conpetitors that abide by thier agreements.
Unfortunately, there is unlikely to be any such thing in this case because - unlike say JK Rowlings - WotC isn't itself a 800 lb gorilla that can really make believable threats. It's in WotC's interest to get its products out to the widest number of distributors, and threatening to pull the Magic cards, minatures, and books from a particular distributor and giving a competitor more exclusive distribution rights isn't likely to help WotC's interests. In plain fact, the profits WotC stands to loose because someone broke the release date agreement are probably smaller than it would stand to lose by going after the unethical distributors and risking its distribution agreements.
Probably WotC's best bet is to do what fan authors like Dickens and Tolkien have done in the past - make a direct appeal to thier readers not to purchase from the offending publisher/distributor. This often works surprisingly well, because the fans generally have a relationship of respect with the author that they don't have with the distributor.
In this case, no one need be mentioned by name. A simple appeal not to buy from a seller who breaks the release date should work. It won't stop everyone, but it will impact sales enough to make breaking the release date risky.
Of course, if they were serious about this, WotC surely would have had a campaign in place before the release date. Frankly, I don't see alot of sign that WotC cares that much. They certainly should have known it would happen.