While in general I tend to agree with the concept that the units were denominated in USD and refunds would have been that amount, in jjiimm_64's case he was explicitly promised his BTC51 back, with no exclusions. If Hashfast was worried about unlimited liability on the BTC51 in the event that Bitcoin went to $1M, the simple solution would be to just not convert that BTC to USD. Yes that opens up the possibility that BTC would go to 0 and their payment is now worthless, but that's the choice they make when they offered him a full BTC51 refund.