As I list bank owned homes, and have worked with buyers on buying them too, I can say that most of the banks behave in similar fashion with respect to your question.
The standard CAR puchase contract has a default 3 day time to respond to an offer. This number can be changed to something else. It sounds like you have specifed a 30 day closing, but I am assuming the time to respond to the offer is still at the default 3 days. So at 5pm on the third day after you wrote the offer, the offer has offiicially expired.
In practise, this date often slips, and banks will usually do their counter offer "verbally" anyways. So negotiations are happening, but the contractual dates have long sinced passed. After "verbal" acceptance, it may take a week before the bank will sign your purchase contract and you can officially open escrow and get the clock ticking on any contractual time periods. As a listing agent or a buyer's agent, I am not happy with the process, but I guess we have to learn to live with it. In a court of law, I'm sure a buyer could challenge many of the policies and procedures banks are using in both REOs and short sales.