'Positive' discrimination is a smokescreen.
The very nature of discrimination means that one individual or group will be favoured and the other individual or groups will not be favoured. in other words, for every act of positive discrimination, there is an equal measure of negative discrimination. It cannot be any other way.
No matter the rhetoric, quotas are also negative discrimination.
After all, if you say there is a quota of 30% female employees then there is BY DEFINITION, a limit of 70% on male employees*. Likewise race.
There are SO many factors contributing to the racial and gender representations in all different areas of life. In the workforce it includes the fact that African Americans are, statistically, poorer and less educated than white and Asian Americans. Now, that may have its roots in the decades of slavery and repression and racism and neglect but it is still a statistically relevant factor when considering the 'representation' of African Americans in a given industry today.
The point is that representation in employment is the end of a long chain of events and the real work needs to go on improving the earlier links - like education. Focusing on the last bit is very much a case of putting the cart before the horse.
It's an attractive option, however, because it neatly avoids having to deal with any real problems. Far easier to just assume that bosses are racist than that public education funding is falling well behind what is needed - especially in those communities that need it most.
* - For simplicity, let's keep it to sex, rather than gender distinctions.