The difference is that cards in Hearthstone can offer a gameplay advantage. Imagine if people that bought Blizzcon tickets got a special Legendary weapon in WoW. I think you can see the problems that would cause.
Promo cards in real TCG's offer "advantages" too.
This is true, but look at ALL other in-game goodies has given before. They are all vanity items. Mounts, pets, banners, icons. Nothing actually had any affect on the actual game (no better gear or stat increases). Just like the items you can buy from the battle.net store. So far, they have not made any games "Pay-To-Win". They want to give the same opportunity to earn their stats.
That being said, I personally see Hearthstone going away from this, because Hearthstone is free-to-play and they need to make money some how. Currently, you get a special card if you pay for cards in the beta. This is only obtainable from spending money, and I think the in-game card Blizzon gives us will be only obtainable this way and it will be a legit card with good stats.