In both cases, it's a male-led society that's deciding when and where breasts can and should be seen. So, young pert breasts in a "newspaper," posed for the enjoyment of men = good, ordinary breasts being used for their actual purpose and not for the enjoyment of men = bad. Why should men decide what happens with breasts when they don't even have any?
Breasts are functional body parts. They serve the very important purpose of keeping young infants alive. And yet, women are made to feel ashamed and embarrassed at using them for that purpose because they're "sexual" (ie there for men - we shouldn't make men look at them unless we're doing it for their benefit rather than for the benefit of a helpless baby). It's a fucked up situation.
I don't think anyone is really that bothered about women walking down the street topless, for now. The freethenipple thing is about ownership, it's about women saying "you know what, these are my fucking breasts and I can show them or not show them as I bloody well please. If you think they're too "sexual" be seen in public, that's your problem." It's about women saying "no I won't accept that page 3 is a wonderful British institution that we must all smile benignly at while women are thrown out of restaurants for showing a tiny bit of boob while feeding a baby."
There is nothing inherently wrong with a woman posing topless for a photo. Breasts are lovely, the human form is lovely, looking at it is no more "wrong" than looking at a flower or a sunset.
What is wrong is the sneery, misogynistic idea that breasts are shameful and must be kept hidden away, even when a baby absolutely needs them, until men decide they want to ogle them in a paper and then it's fine, in fact it's more than fine, men will actively campaign to make sure they see those breasts. It takes agency and ownership away from women by implying that men get to decide what is right and wrong and their wishes and needs are what is important.