The practice of charging a transaction fee when you charge with a different currency than what is the primary currency of the company is quite normal. The fact that a fee of some sort might be needed for the extra hassle aside, they need to cover for the case that the currency exchange ratio changes from when they set the price, as it would be crazy to keep adjusting it.
For example, I'm in US and spend USD, when I charge in a certain site in China, I have to pay 15% extra.

That said, while the practice is normal and usually expected for most non-global companies, it should be explicitly stated that there is such a charge upfront before the purchase/payment is made (as is the case in the aforementioned site). Granted, it might be hidden in some of fine print.