The terms may be crooked in your mind, but they are not illegal (that I am aware of), and everyone is free to read them BEFORE ordering. And of course Coinabul would still owe them a full refund in USD equivalent if BTC dropped to $1. The BTC price after the time of purchase is completely irrelevant - again, because of the terms that Coinabul lists.
So your argument is essentially that so long as Coinabul's terms were knowable to the customer prior to the order being placed and the customer placed the order nonetheless, Coinabul cannot be scamming if they follow their own terms.
Coinabul's TOS says "If Coinabul is held liable to you for a transaction, liability will be limited to either the amount which you paid during the transaction or $100; whichever is greater."
So if I place an order for $1,000 worth of gold, Coinabul opts not to send me the gold for no reason, I sue them, get a $1,000 judgment and they pay me $100, that wouldn't be Coinabul scamming?
It is, unfortunately, fairly routine for companies to have terms of service that say "We can do whatever we want and your only recourse is to ask us nicely to do something else". While companies think this provides them with all kinds of legal protection, what it actually does is cause people to have no choice but to ignore the terms when determining if the company acted responsibly or not. Coinabul's terms are these kinds of terms -- many of them contain "we can do whatever we want and you have to take it " clauses, such as this beauty: "Coinabul is not liable for any errors, negligence, or inability to fulfill any orders."
I'm not saying Coinabul shouldn't have this kind of ToS. While I think it's idiotic, I understand that lots of companies do this, often as a result of specific legal advice. And I'm not a lawyer. But it forces people to basically ignore the Tos when deciding whether to buy from the company or evaluating whether the company acted reasonably.
The reason nobody bothers to read a ToS is because they all say the same thing -- we can do whatever we want.
And here's where it hurts you -- this would be a reasonable term of service. So if Coinabul could argue that its ToS was reasonable and that customers agreed that they were treated fairly so long as Coinabul complied with its ToS, I'd agree 100%. However, because Coinabul's ToS is of the "we can do whatever we want" type, "we complied with our ToS" can't provide a defense against an accusation of scamming. So "make your ToS make you liable for nothing" may be good legal advice, but it fails in the court of public opinion because it prevents you from being able to point to your ToS to justify your conduct. An unreasonable agreement can't justify anything.
Like I said, their terms are not illegal (that I know of). I know there are limits on ToS, and I'm not really familiar with laws surrounding this, but the fact that credit card refunds can differ based on currency exchange rates is really all the proof that I need to see that it is a normal practice for a business to have a single currency of denomination, and that a business is generally free to choose what currency to denominate in.
I could certainly be wrong too.
And I'm not saying this is the best thing for Coinabul either, nor am I trying to defend their business practice or terms of service. I am strictly pointing out the legality of it, and that they would not fit the legal definition of a scam as far as I am aware, provided they actually do refund the customer in USD equivalent.
From an ethical and business reputation standpoint, I think the right thing to do would be to refund the entire BTC balance (if they never sold the BTC) or refund the USD equivalent, shipping including, of the BTC at the time that the order was made (if they did sell the BTC). Just my opinion, but I would find either of these to be an acceptable resolution, and I believe OP would as well.