The word "illegal" is being thrown around here without abandon. Reminds me of my first week of law school before the Profs kicked ass.
A street date is a contractual agreement between MS and its distributors. Breaking it is not in violation of any criminal statute, even by the retailer. It's a violation of a civil contract and MS could sue, seek liquidated penalties if the contract provides or in some cases seek an injunction against the retailer prohibiting any more early sales (this last equitable remedy is hard to obtain because MS typically has a remedy at law). The End User is not a party to the agreement. There is no privity of contract with the End User. MS can't hold the End User in breach of that contract and sue them.
What MS can typically do is what they've done: Suspend access to their online service. I say typically because I don't know what the End User Agreement for Live says, but I bet it gives MS great leeway in banning End Users for things like this. Whether it's smart business practice to do so in this situation is another whole discussion altogether. But can MS do it without incurring civil liability to the End User: Likely yes. And let's face it, it's not like the End User is going to spend the money to sue MS. It's more likely MS wouldn't take this action to avoid bad press, assuming the End User did buy a legitimate copy.
As to those casting the first stone on the End User, I am willing to bet my copy if Halo 4 coming from newegg next week that most if not all of you have bought "grey market" merchandise of some sort (look it up) at even legitimate retailers that was not intended for sale in your area. This happens all the time and it's in violation of distribution agreements I assure you. I suspect 99% of you would have continued to purchase the item even knowing this. The companies make their money either way so don't feel too bad for them (piracy... Now that's another issue)