At the very least, the law is applicable only to "Involuntary Denied Boarding". They can offer whatever they want for volunteers.
Delta has just announced they will pay up to 10k.
http://www.cbsnews.com/news/delta-offer-10000-to-flyers-give...
The _airline's_ hands were not tied. But the hands of the agents on the spot might have been tied, by airline policy. Because said policy may well have said they can't offer more than the required legal thing.
I suppose an excellent reason for an airline to offer more than $1350 would be to avoid losing $1.6 billion off its stock price.