With the greatest of respect you took a gamble and lost. You hedged, in full knowledge that your currency would devalue, and Blizzard, who probably had a lot of people doing that, decided to act (which is well within the terms if you’d bothered to read them). You can’t expect a company to make a loss to make up for a whole country’s currency issues when its market cap is in the tens of billions of dollars versus a currency with trillions.
While it’s fair to say you didn’t know what Blizzard would do at the time you made the balance purchase, you didn’t actually buy Euros, you bought a game currency notionally denominated in Euros. The difference being if you’d had the Euros you could have freely swapped them or withdrawn them. But your battle.net balance has no cash value. Battle.net isn’t a bank.
If you’d not loaded the money onto your battle.net balance, then when Diablo came out you’d still have had to have paid more in your own currency to buy it at that time than at some earlier point if the currency devalues in the interim. You knew that, hence why you bought the balance when you did.
Could Blizzard have given more notice, or handled it better sure, but in forex you wouldn’t expect to get the same back as you spent if the relative values changed in between so why would you expect Blizzard to?
As an active Account holder, you may participate in the Battle.net Balance service (“Battle.net Balance”). Battle.net Balance can only be used to obtain certain products and services offered by Blizzard; it has no cash value. Blizzard grants you a limited license to acquire, use, and redeem Battle.net Balance pursuant to the terms of this Agreement. Regardless of how it is acquired, Battle.net Balance is non-transferable to another person or Account, does not accrue interest, is not insured by the Federal Deposit Insurance Corporation (FDIC), and, unless otherwise required by law or permitted by this Agreement, is not redeemable or refundable for any sum of money, monetary value, or anything of value, from Blizzard at any time. The Battle.net Balance does not have an equivalent value in real currency and does not act as a substitute for real currency. You acknowledge and agree that we may revise or take action that impacts the perceived value or purchase price of the Battle.net Balance at any time except as prohibited by applicable law. The Battle.net Balance may terminate in accordance with the terms and conditions of this Agreement, when we cease providing the Software, or this Agreement is otherwise terminated. Battle.net Balance does not constitute or confer upon you any personal property right. Battle.net Balance is not a bank account. And, while you can register and play on multiple Accounts, you are not allowed to have more than three (3) Accounts with Battle.net Balance.