Correct me if I'm wrong, but I don't recall any instance where there was an event distribution that required you to pay exclusively to obtain a mythical Pokemon. There have been promotions for Pokemon Bank and stuff where you can get an event Pokemon if you buy the service, but you're not really buying the Pokemon... you're buying the service. It's not, "hey, pay $20 a year for this Pokemon" it's "hey, if you get this service that can be useful for storing Pokemon, you can have this event Pokemon".
Also, what is the logic behind "I cannot use this Pokemon to battle NPCs, who do not care which Pokemon I have, so there is no reason whatsoever that this Pokemon should exist"? I might be wrong, but aren't the battle facilities just for battling NPCs? And, even if I am wrong and they aren't just for battling NPCs, just because you can't use them to battle people doesn't mean that they shouldn't exist.