It's in the ideal orbit for Baikonur. It's much, much easier for a low-latitude launch site to reach a high-inclination orbit than vice versa. This is especially true for Baikonur, where the available inclinations are restricted by human habitation and foreign countries downrange. Hence the minimum inclination is 51.6 degrees (which is where the ISS is) despite the launch pads being at 46 degrees latitude.
You are correct, I had remembered the details wrong- ISS and Mir and Salyut 7 all have the same orbital inclination, so that's clearly the best orbit for Baikonur. So if you were doing your mass launches from Baikonur you would not pay much of a penalty. From KSC, though, you'd almost certainly want a lower inclination to get more payload up.
Really, you pay the same penalty launching to 51.6 degrees regardless of whether it's Baikonur or KSC. But you're totally right, you can do even better by launching to lower inclinations out of KSC, which can go down to 28.6 degrees.