I don't understand the idea of "...an ethical responsibility to work full time" at all. Why shouldn't the amount of labour you want to sell be open to negotiat

I don't understand the idea of "...an ethical responsibility to work full time" at all. Why shouldn't the amount of labour you want to sell be open to negotiation? The fact that it's relatively hard to find "normal jobs" in my field in formats other than full time is one of the factors that makes me seriously consider the jump to freelancing.