I think some people are against positive discrimination because they think it is incompatible with hiring people on merit. I'm not convinced that it is necessarily incompatible in a way that matters though.
Of course it is easy to think of ways in which positive discrimination would preclude a meritocratic outcome. I'm just sceptical that such thought experiments really reflect best practice in the real world.
There has been some debate recently regarding a popular uk television comedy programme called Mock the Week which has been criticised for mostly hiring all male panels. In their defense they argued that the percentage of professional female comedians is low, so in demographic terms the percentage of women they hire is more than representative. They also said they were against positive discrimination because they believed in hiring on merit.
I think it is the last point there where a false dichotomy has crept in. In the real world it is just not the case that merit is the only criterion used and nor is it the case that assessment of merit is free from cultural bias. Instead I think it would be more useful to consider merit as a threshold criterion. That way, you can look at the pool of candidates you have who meet the threshhold and decide what other criteria to apply. If there are still sufficient numbers in your pool to choose from after all criteria have been met then it is not the case that merit has been trumped by other considerations.