I simultaneously feel like, in this particular case, neither are the corporations themselves the cultural and moral enforcers, nor is deciding on where to draw the line on which norms to enforce relevant.

The former because the rights groups which criticize the corporations are the ones driving the supposed change and enforcing their own morality. Rights groups are probably better moral arbiters than corporations, on account of there being less conflict of interest. And the latter because, while the line can definitely be very hard to trace in various, more ambiguous situations, I really don't think this specific case is one of those. It feels like a slippery slope argument to me, specifically: "If we enforce this, what else are we going to end up enforcing?"