That's because it was actually warping the Arena meta. After the first day or so of Arena, people straight up stopped drafting dragons. There are a couple that are still worth it (Tyrant is too good to pass up even with Poacher around, and Alexstrasza is still an "I win" button), but for the most part you played around Poacher to an extreme degree. I picked up two Poachers in my Paladin deck that I drafted this morning, and every game I've played them they have been crushing. A lot of Arena decks just can't come back from a turn 4 8/8 Rush. Or a turn 8 16/16 Rush.
The change being accidental makes it a lot worse - anyone who drafted a deck from before the change is utterly screwed, because their deck is going to be full of dragons. Anyone still drafting but not seeing a Poacher is also screwed, because the change wasn't announced and so people won't know about it.
Taking Poacher out made an immediate and noticeable improvement in Arena. You could actually draft dragons again. I've had a LOT more fun since it was taken out.