Honestly, I don't think a lot of them feel guilty. I sure as hell don't when I give DD a FF. I'm feeding my child so she doesn't starve, why on earth would anyone feel guilty about that? You're not offering them a line of cocaine, it's a reputable source of infant food!
There are alot of Mum's that DO feel guilty for feeding their babies formula and your right, they shouldn't, especially if they know they did everything they could to try to BF