I don't know if this is a lactivist thing, although as a PP pointed out, if you can go topless legally, you should be able to nurse legally. But I don't think it's quite that easy.
Let me be clear: I have absolutely nothing against public nudity - either gender - and I think that north americans are generally far too uptight about bodies all over. I personally have been known to frequent the naturist beach near here.
But, I also think that one of the best ways to further the lactivist cause is to try to get the idea into people's heads that breasts have two distinct purposes - lactation and sexual fun. Because honestly, I don't think that people have issues with NIP because of the exposed breast, I think it's because there's a baby attached to an exposed breast. KWIM? Like, if there were a bunch of guys at a strip club, and one of the strippers sat down in the corner and started breastfeeding, most of the oglers would be icked out. (Not that I'm advocating having children in strip clubs. Or the existence of strip clubs anyway.)
The underlying cause of issues with NIP isn't - in most cases - a puritanical approach to body parts, it's a lack of ability to internalize a duality like that. So I don't know that acceptance of toplessness in women would equate to acceptance of breastfeeding in public, even if it were to happen.
And - to be perfectly frank - I think trying to desexualize breasts (which I assume is the goal of the topless movement?) is a bit of a lost cause. The current theory is that humans have breasts regardless of lactation status (unlike most non-ungulate other mammals) because it's a sexually selected trait, most likely because breasts present a similar aspect as does a bum, so men would be more apt to have sex face-to-face, which theoretically increases emotional bonding and stability of a pair-bond, and also increases (generally) female enjoyment of sex. Sexual enjoyment of breasts is pretty universal, and while I would certainly like to see people comfortable enough with themselves to be able to enjoy looking at everyone's breasts all the time, that's pretty ambitious. I DO think it's within the realm of possibility, especially if young people are exposed to it regularly, to accept lactating breasts as non-sexual - it's a specific, logical exception along the same lines as "NO PETS ALLOWED - guide dogs permitted". We can handle that sort of thing.