It may well be the case that HR dept's send out Dear John's. But when an offer is accepted it then constitutes a contract and rights and obligations are bestowed on both parties.
Informing one party before the starting date that the other party no longer wishes to 'perform' is an anticipatory breach of contract.
If I am a landscaper and agree to your offer of building you a monoblock driveway for �4000, with the work beginning on November 25, 2007, that is a legally binding contract. If either side changes their mind then a court can award the party in breach to pay damages or force them to perform.
Now most employers won't bother taking legal action, but they could, and they'd be entitled to do so.