I have no idea, to be honest. If the contract ends prematurely, then a half refund sounds fair to me - otherwise you're getting something for nothing.
I'd assume that Mongoose knows what it's doing, legally.
I'm suprised to hear that there are countries which would handle that in a different way. What if, for example, the situation was reversed? Of you had paid short by a few pennies for some reason (by mistake, for example); would you think it OK if, as you had only partially fulfilled the contract, they got to keep your money and deliver you nothing? Because that's what you're asking them to do - you get to keep their magazine and you pay them nothing.
In the latter case, what any company would do would be to require you to come up with the missing amount of money. If you couldn't do that for any reason, they wouldn't be able to just keep the money you'd sent them and ask for the mags back. Similarly, since they can't come up witht he missing magazines, you can't just keep what they've given you and ask for your money back. You can get back the cost of the mags you didn't receive, though.
In fact, take it further - what if it was a year sub to Dragon and, for some reason, Paizo couldn't deliver the final (12th) issue it owed you. Would you feel you should have all your money back AND the 11 magazines? Or that they recompensed you for the mag you didn't get?
