Instagram is censoring women's nipples because it can lead to a slippery slope regarding their content.
Once you allow artish stuff like Bellemere's where do you draw the line before full-on porn is possible? And they want to be kids friendly as much as possible.
Or to put it more accurately, they want to be parents friendly as mush as possible.
That's it. This isn't about freedoms or other BS like that, it's about maximizing user count and hence value of the company.
So I get it where are they coming from. I don't use FB but as far as I know they have the same policy (and they own IG).
All in all, I don't have problem with their censorship. If Bellemere wants to have his stuff uncensored, he can easily set up his own website like T. Richardson and post there whatever he wants.