Has it always been like that or just since the TLC show aired ?
It seems to have been like this in the DC Metro area for as long as I can remember. I won't say it is absolutely true that no store here ever doubles coupons, but if it does happen, it is rare enough, or at a store out of my way enough, for me not to notice.
Without doubling, most coupons are worthless, because even with the discounted price the store brand of the item is almost always cheaper.