I believe it's done because the gate agents are hampered by policies on what they can give as vounchers. Since they can't raise the voucher value to make it more attractive, they toss in the seat up front.
I love my domestic upgrades as much as the next person, but still, I understand the situation. Heck, I've even been known to take the double bump plus a seat booked into "A" myself when I lived in FLL.
Steve