I'm not sure I agree with that. Coinabul's policy states that their refunds will be in USD equivalent, not BTC.
Now, if you want to argue the potential for gain on other investments in the timeframe that Coinabul held on to his money (i.e., interest), I think that is a fair argument to make. But it would be up to a court of law to determine specific damages, and it most assuredly would not follow the BTC price.
I'm shocked that some people don't see the theft transpiring. You speak of bitcoin as if it holds no value as a commodity and currency independent of USD. I never would've thought there'd even be a handful of people arguing on behalf of USD let alone a "court of law's" undoubtedly-twisted interpretation of bitcoin. According to those insinuations, then if the USD value of bitcoins dropped to $1, Coinabul would still owe them the spot price of gold?
No. They would owe them the 90.78 BTC still. If Coinabul's "terms" are to rip people off then those are crooked terms. Right. Let's start an international store, take bitcoins, give nothing, watch the bitcoin value go up, then decide to hold on to part of the coins. Nice 'operation' there.