No they are obligated to give full btc refunds because it was specifically stated in the ToS and also confirmed in an email to support staff.
Perhaps morally so, and perhaps in some non-US jurisdictions (and maybe a state or two that has wildly different laws on contractual remedies), but in nearly every US state, the remedy for a violation of a contractual duty to deliver a fungible good is the market value of that good at breach, and unless the parties agree and execute otherwise, that remedy is in legal tender.
It is no doubt a complicated legal situation. However the fact that HF promised full btc refunds and changed their mind later when the value of btc went up is without a doubt bullshit. Considering many people only ordered with hashfast because of these terms along with a supposed guaranteed refund if delays extended past dec 31 which they did.
I doubt that the US law protects btc consumers from this type of BS even though some have apparently sued BFL for their btc and won(although don't think they collected)