It is piracy. But how bad is piracy when the alternative is nothing?
If Microsoft has zero plans to bring over halo online, then they have already forgone any profit that the west could have given them for the game. They said "nah, we don't want their money" they could have easily said "halo online is testing and launching initially in Russia, then worldwide at a later date" but they didn't.
If Microsoft was already getting no money from me (that I would have sincerely GLADLY given them to buy content and show the pc crowd is a lucrative, worthwhile market and that they should bring more halo games to the platform) then I see no reason in playing a free fan-translation. The game itself is free already, now I get to play it.
The alternative is that these hackers leave the microtransactions in the game, and take the money for themselves. Which is entirely amoral. So them giving a free game to a non-audience who wants it and stripping the microtransactions so the game is playable is fine by me.
Nintendo has not released Mother 3 in the US. They likely won't. Yet, it's been played by a ton of people in English thanks to the hard-working fans who translated the whole game themselves and released it as a patch. The moment that Nintendo releases Earthbound 2 here in the states I'll happily buy it, same if Microsoft releases halo online here.
But since Microsoft dropped the ball on even pretending like this game would get released worldwide, people have found an internal build and are doing it themselves. They're making no profit off this. You could argue that Microsoft could be losing money, but since they did not state that they were even considering bringing the game over, they implied that the western market will never get it. So they can't lose money on something that they never intended to try and make money off of.