In all fairness, one other possibility for remastering isn't to re-sell another copy to those who have already bought the DVD set, but to entice new, first time buyers who may not want to invest in an "older" format for a big, 12-season set...
I don't know one way or another, but I think sometimes conspiricy theorists assume that every Blu-Ray re-release is to gouge the customer. Some, I'm sure, are. But other times it might be done to future-proof their body of work and keep it viable in the market.
