There is no feasible way for them to know if someone bought from an unauthorized retailer other than rough key location and the users country.
Well, they know the key was initially sold in region A, and they know the key was activated in region B. If there aren't any resellers in region A who are authorized to sell to customers in region B, then that seems like a pretty feasible way to reach that conclusion.
Is the world becoming dumb or am I the only one to see the obvious theft here ? they get the money, you can't use your product and will be forced to buy another one ...
That's life. If you pay for a service and violate the terms, they can cancel your service without obligation to refund your money. The problem with your analogy is that it involves a product, while Steam games operate under the "service" model.
The software industry operates in it's own special universe. Companies will blatantly and deliberately mistreat their own paying customers simply because they have the power to do so with little to no real consequence.
There's actually nothing unusual about this. It would be nice if Activision would go after the unauthorized distributors and then refund the customers, but generally speaking, companies don't do these things unless it's best for their bottom lines or if they are otherwise obligated to. Fact is, the customers broke the ToS that they agreed to willingly, and even if they didn't read it, they should have known better than to buy a "discount" key from some website without any reputation. Activision won't win any loyalty with their behavior, but they don't owe anyone any courtesies either.
Also, suppose Activision does refund everyone's money. Then Activision loses out - whatever amount the customers paid, Activision received less than that in the first place, and the difference went to the unauthorized resellers.