Social justice in moderation is a good thing. It's actually because of social justice that black people and women can vote, women can hold jobs and get paid the same as men, gay people are no longer dragged into the streets and shot at, and children are no longer sent into dangerous factories. However, there are people who are fighting for more, and these are the radical leftists, and their arguments that women should get paid more, gay people should talk to children and teach them a new type of 'normal', and pushing for an increase in minimum wage (which can lead to companies increasing the cost of product) are met with debate and heavy discussion. But we could say the same thing about far right conservatives fight for the criminalization of gays, the right that every single person to bear arms, twitching or not, to force biblical ideals to children whether they or their families like it or not, and for the government to shoot at illegal immigrants at the border without hesitation. These ideas are also extreme and met with heavy backlash, but we don't really talk about those guys.
Point is, being a political extreme on either side is problematic, and both political extremes are not compatible with core Christian principles.