Most states (though not all) have laws stating that a woman is free to breastfeed her child anywhere, public or private, that she and her child are otherwise allowed to be. However, with the exception of a few states (CT, DC, NJ, VT, & WA), there exist no enforcement provisions. This means that as soon as a woman who is legally breastfeeding her child is asked by a business owner or security to leave, she is then trespassing.
Other countries (i.e. Taiwan, the UK) have fines for those who would interfere with a woman's right to breastfeed; the US should as well.
A woman should not be forced to hide, whether under a fabric cover, in the restroom, or in her car, simply because she elects to breastfeed her child. At its core, this is an issue of discrimination and should be treated as such.