I think it's more that there are a lot of things they have to keep track of for each publisher / author regardless of income. When it's very low income coming in, like me, it gets to a point where they are probably running the account at a loss. Drivethru was set up much more with small publishers in mind, and I think with much better automation. My take there is regularly much better than on Warehouse 23, so it isn't a huge loss to lose that market. There used to be a tiny reduction in Drivethru's cut if they were the only distributor, I'll have to check if that still applied once Warehouse 23 have pulled my stuff.