Like any other game, web site, etc. You take advantage of an unexpected software issue that resulted in an unfair advantage, the company has the full right to undo as much of that damage as is reasonable.
And if you got more than 4 warehouses, you were blatantly giggling inanely at yourself that you found a knowing defect that you can take advantage of to get more loot.
BB is being plenty fair with their policy. They'd be in line if they deducted the cost of the warehouse in equivalent gems too.
As for those who want a refund... Get serious... You paid, you used up time playing. Can you return the time you spent playing? Of course not. So don't be going making silly requests.