Because they are exchanging their labor and their bodies for money and then turning that around to somehow present as a part of their anti-capitalist radicalism. Sex work is just another form of exploitation and should be seen as such even by the people who think it's no different from working retail. After all, aren't you fighting for a revolution after which nobody has to work retail either?
Also, the anarchist movement on the whole tends to be very good about women's issues and very pro-feminist. It pains me to see well-read radical people who identify as anarcha-feminists and feminist allies doing something so highly anti-woman and pro-patriarchy as supporting and endorsing sex work.