This is a known practice, and one that a lot of people are unhappy about. We suspect that the TTB will be cracking down quite harshly on this in the near future; forcing a named distiller statement on the label.
That said... not all of the whiskey, rye, or bourbon that MGP produces is bad - some of its aged production is quite good, and companies like Angels Envy which further age it (basically double oak or quarter casking) can produce a unique and high quality product.
My problem with this is from the deception; imagine if it turned out that Dogfish Head and Stone brewing were actually buying their beer from MillerCoors! This is an unfair practice that puts actual craft distillers (some of my clients!) at a huge disadvantage. A named distillery requirement would go a long way to correcting this.
[1] www.Gastrograph.com