It is true that in most states, breastfeeding foster babies is considered illegal. Babies have been removed from homes when social workers have found out that foster mothers were breastfeeding. Other families have been given stern warnings. And some families breastfeed their foster babies without consequence.
I disagree, though, with the general sentiment on this thread that breastfeeding a foster baby would be so disastrous, aside from the potential legal consequences (i.e. if the baby were to be removed from the home for this reason alone). I think perhaps that some of the fear comes from the way breastfeeding is viewed in our country, as being something that should be kept private, done discreetly, or something that might gross people out due to the inappropriate intimacy. Why has our society become one that would see feeding someone else's human baby human milk as potentially "unethical," while feeding someone else's human baby cow/soy milk is just fine? Back in the day, when wet-nurses were common, no one would have blinked an eye at the idea of a lactating woman breastfeeding a baby whose mother was unavailable at the time.
The families I know of who have breastfed their foster babies have not exclusively breastfed them. They have made sure that the babies were comfortable taking bottles and drinking formula, so that if the babies were to suddenly move to a new home, they wouldn't be without a familiar source of nutrition.
And if, as some seem to have implied, breastfeeding would intensify the bond between foster mother and baby, I say all the better! What foster babies need is to bond with their foster parents. To form strong attachments to their caregivers that will enable them to continue to form strong attachments to other people in the future. I do not agree that the loss of a baby's foster mother would necessarily be more devastating if the baby had been breastfeeding. It is a loss that will be devastating no matter what. And the better attached a baby is (whether he/she has been breastfed or bottle-fed), the more willing and ready the baby will be to attach to his/her new caregivers.
It is true that as foster parents, you do not have the same rights to make choices for your foster children that you do with your biological and/or adopted children. You can't decide to take a vacation without getting permission first. You can't hire a baby-sitter without having the sitter go through a background check. You can't always make the medical decisions that you would choose to make. But, for the most part, you CAN choose to feed your foster children the food you would like them to eat. I fail to see the difference between offering a whole foods diet to a child who is used to eating processed food, and offering breastmilk to a baby who would otherwise be drinking formula (this metaphor IS NOT meant to compare processed food with formula, but rather to point out that foster parents change the diets of their foster children all the time). Yes, these are the kinds of choices that parents make for their own children, but parents of children who are in foster care are not there to make the choices about what foods their kids are getting in their foster homes, and thus it becomes the duty of the foster parents.
For the record, I did not breastfeed our newborn foster baby during the brief time he was with us. I was too scared (for a variety of reasons). And because our foster baby had been breastfeeding with his mother before he came to us, I was the first one to offer him formula. When he moved on to his adoptive home a couple weeks later, I deeply regretted my decision not to nurse him at least a little. I wished that I would have been strong enough to give him all of myself, to offer him everything I had to give.
It is a complicated issue, to be sure. The legal component certainly cannot be ignored. But, legalities aside, I really fail to see how breastfeeding any baby could be harmful (of course assuming that the breastfeeding woman and baby are healthy).