Alliance race balance says otherwise. You're just making shit up to justify your claims.
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What would free tranfers achieve when nobody takes the offer because the opportunity cost isn't worth it? The problem isn't money. The problem is that it wouldn't work. Make the cost of faction transfers to Horde to high and people will just level up new Horde characters instead.
The only way they could fix the imbalance this way would be forced transfers, and that's unlikely to be a can of worms they'd like to open.