I doubt it has anything to do with becoming an ITD. First, if you become an ITD you don't get much of a deal, and then you have to pay their inflated shipping/freight, which basically negates the wholesale pricing. Second, you have no idea when or if you will get what you order. I have had orders open for 8 months, and they will send 1 of this, 3 of that, 25 of this, etc. Third, it is very tough to become an ITD as you need a BM location. The minimum order requirements are easy, but they removed the option for online only retailers a year or two back, and I'm quite certain it is still that way. So there is no reason for a reseller to want to buy direct. Not to mention almost 100% of the resellers fit the profile of not being able to meet Lego's minimum requirements of needing a physical store front.
I believe they don't like resellers for the simple fact that they want the product to be available to the end user. The end user who might walk in a buy one of this, or that, and a few of these, etc. And before you know it you have a shopper who has spent more time in the store and more money on regularly priced items. As compared to the reseller who is looking for only the discounted/hot items, and spends 1 minute in the store. You get no add on purchases from a reseller, and they can only really make your brand look bad (oos instances and inflated prices, thus damaging brand image down the road).
It's the same as any other retailer that dislikes resellers. It doesn't make a lot of financial sense to ban somebody who spends more money than 99% of the people who shop there but they still do. And they all do it. To date I'm pretty sure the only stores I have not been banned/blacklisted from is Sam's, Lego (which is miraculous, but probably has to do with how much I used to order from back in the phone special days) and most of the overseas places.