To me this seems a very extreme position. This is a spectrum, surely we are all going to draw the line at different places, but I think saying they are never ever obligated morally to sell something, seems short sighted to me. Obviously even if books are available elsewhere, not being on major platforms like amazon, is going to impact what books actually get made. If amazon says tomorrow, "No books covering sexual orientation or LGTQ issues" I would take issue and say they have a moral obligation to not exclude books on that basis. Because that is going to impact both peoples access to such books and whether such books even get made in the first place (and it is also just a bad reason morally for them not carrying the book). Or if they took down the bible or the Quran I would say the same thing.