> the only thing removing category distinctions here will do for you is give license to treat any organism however you wish

This is only true if what stops you from treating certain organisms in a certain way are these objective category distinctions (rather than the approximate ones your brain makes), but I don't think that's generally how it works. People who wish to harm others will harm others regardless, the only thing that will stop them is consequences. On the other hand, people who don't wish harm to others will not harm others regardless of whether they believe objective categories or objective morality exists.