Tell that to anyone who:
Does not own a compatible wifi connection.
Does not live near any stores which offer the event.
Is not aware of how to track upcoming events.
Purchased the game, still at full price, after the event expired.
Lives in an area that rarely gets any events like, say, Australia, New Zealand or pretty much anyone in the "other" region.
Unfairness is defined as a lack of equality and it seems to me that a Japanese (or hell, even an American) person living in the city has a massive advantage over someone who lives in rural Australia. That is why it's unfair, because some individuals are able to obtain said pokemon far more easily all because of some irrelevent traits such as location, router and knowledge of 3rd party pokemon sites.
And while this doesn't really relate to fairness in any way, giving out the most powerful pokemon in the game for free with zero effort required is just boring game design and breaks the player's immersion.
When I look at my Kyurem I remember everything I had to do to catch it. I remember chasing Team Plasma, I remember fighting Ghetsis, I remember being forced to defeat it, I remember all the time I spent, all pokeballs I wasted trying capture it.
But when I look at my Genesect all I remember is talking to the mail man and sticking the pokemon in the PC, never to be used again.