The reason for this is, they failed to deliver, that is not under debate. But they are assuming that someone who was long on BTC as a speculator, is still long on BTC.
In the latest edit Brian from coinbase mentioned that they are now honoring today's rate, which is a step in the right direction, and has subsequently save the OP substantial losses, but this does not absolve them of the original screw-up.
I doubt the OP will complain. If I were in his shoes I would personally request a complete reversal even if I wanted to buy at today's prices, and manually make a new request.
Any seasoned/experienced business would do this.
Example: If amazon fails to deliver/the shipped item is lost in transit, they reverse the credit card transaction and request you to re-order if you still want to purchase.
This sort of settlement process is a scary thing to throw very much money at.
What they probably should have done is just reversed the transaction entirely, but if I was deciding this, barring some agreement on delivery time, Coinbase would be mostly blameless.