It is a little more simple then any of the above answers....
Bulk propane filling is being slowly phased out in Canada and the United States. The preferred 'safe' way for filling propane tanks now is the exchange program where the empty tank is returned to the propane yard and inspected / tested / refilled there.
Due to many accidents at the local gas station with LP tanks and filling stations, lack of proper inspection of customer owned tanks, and loss of too much propane vapor into the air.... many safety organisations are attempting to ban such stations. In Canada it is the TSSA that is making it very difficult to have a propane tank refilled by the customer, they make the annual fees and insurance costs way too high for most businesses to justify. Now we have to bring our nearly empty (never fully empty) tanks in for exchange and pay for a new full tank (always losing just a little bit of money)
The 30 lb tanks are not typically available for exchange, just the 20 lb tanks.