Just because the tanks are 'expired' doesn't mean they have NO pressure left in them. Taking a band saw to a pressurized metallic structure seems... a most unwise venture. And recycling that is going to be a LOT more complicated than recycling soda cans. For one thing, I believe it's going to be a different alloy (4% copper, if memory serves), which is going to change things significantly.
If it would have been profitable for the fire department to recycle them, I'm pretty sure they would have just recycled them.