There are already a lot of answers, but I have to add mine.
I agree that this is a double standard. Here's the reason, in my own case: Health care professionals, family members and friends see encouraging formula as being sure the baby has enough to eat. Because these people have little to no experience with breastfeeding, they think babies should ALL be big and fat and sleep for hours on end between feedings. This was not the case with my daughter. My mom was constantly saying "Jenny, I'D go on and give her a bottle. Just ONE bottle!" It drove me nuts and killed any confidence I may have had. We did end up supplementing, and I hated it. I always thought my breastmilk wasn't enough because my daughter was so tiny. We stupidly introduced solids too early to try and get some weight on her (I know, I know--stupid, and it didn't work). But guess what? Now she'll be two years old this month and eats basically whatever we eat, and she's STILL tiny! It's just how God made her. Some babies are chunky, some are not. It usually has nothing to do with your breastmilk.
Then, for some reason, most people think that encouraging a mom to breastfeed will make her feel "guilty" if she doesn't want to do it. Guilt comes from one's own conscience. I breastfed for 21 months and, although I was irritated that people judged me for it out of ignorance, I never felt guilty because I knew I was doing the right thing! And anyway, do people not realize how hurtful it is to a mom when she's trying to make a go of breastfeeding and is constantly told, especially by the doctor, that she SHOULD be supplementing (which she knows will ruin her supply)? She's between a rock and a hard place. She has no choices. Mothers have had social services called on them for refusing to supplement in some of these situations, which is downright terrifying! So, in comparison, forgive me if I don't feel much sympathy for moms who choose formula without even trying to breastfeed and want to be patted on the back for it.
One other thing. I get tired of women saying "I hate when people ask me if I'm going to breastfeed! That's personal!" That is NOT personal; unless you intend to hide your formula and feed baby in the closet, everyone is going to know anyway. It just makes them uncomfortable to have their parenting decisions called into question. I am pregnant with my second daughter now, and I don't mind telling people I'm going to breastfeed. Maybe it's because I am proud of my decision. I like to talk to other moms about breastfeeding to see if we have anything in common--not to start a fight.
And also, as I'm sure you know, hospitals and doctors' offices get compensated for pushing certain brands of formula. For encouraging a woman to breastfeed, they get nothing but the satisfaction of trying to do the right thing AND a pregnant woman who feels "guilty" and has hurt feelings. Most of them take the easy road.