I would imagine that producing the DVDs requires some capital. Having a presale w/reduced prices might provide the financing to get the project off the ground. Production costs must be substantial, particularly given the high quality of Cathe's overall product (sets, filming, sound, music, DVD chaptering, premixes, etc.). Then the higher prices later on provide more of a profit. I'm just guessing here---I could be way off base.
In any event, I've never been disappointed w/anything I purchased in the initial, lowest price presale offer from Cathe. Let's face it: it's rare that you have to pay full price here. There are coupons every month, deals of the day, etc.