Two points:
- In PTR they can gem/reforge/enchant how they want, and they are also testing the class changes; also with premade characters not nescessairly importing their own chars
- They can play freely the bosses, but still encounters can have minor tunning or adjustments
If they can play freely, they play how they want, the time they want.
When it goes live, it's their own "live" toons and it's a RACE, to see the fastest - and so the best - raiding guild.
Complaining about that seems like saying the same thing in a car race: "What? They do a Free Practice before the race? So they know the car, the track and the opponents before hand? What's the point in having a Race to see who win?".
For you (and me), spectators, it could be like "more exciting". But totally unfair to that players who take that "business" seriously.
Not only that: doing this way Blizzard keep the race clean. If they made the stuff "top secret", it could always be the suspicion of someone/guild beeing favoured by having contact to someone inside Blizzard. I can imagine someone creating a post titled "The Lei Shen Scandal" or "Firelandsgate". And, as always, yelling something like "Blizzard is a joke".