I think the point of the article and general consensus here is that their missions should not overlap as much as they do.
Or as the ACLU puts it, on why they defend abhorrent groups:
"The ACLU is frequently asked to explain its defense of certain people or groups — particularly controversial and unpopular entities such as the Ku Klux Klan, the Nation of Islam, and the National Socialist Party of America. We do not defend them because we agree with them. Rather we defend their right to free expression and free assembly.
Historically, the people whose opinions are the most controversial or extreme are the people whose rights are most often threatened. Once the government has the power to violate one person’s rights, it can use that power against everyone. We work to stop the erosion of civil liberties before it’s too late." https://www.aclu.org/about-aclu
Which is somewhat different than the SPLC:
"The SPLC is a catalyst for racial justice in the South and beyond, working in partnership with communities to dismantle white supremacy, strengthen intersectional movements, and advance the human rights of all people."https://www.splcenter.org/about
Or as the ACLU puts it, on why they defend abhorrent groups:
"The ACLU is frequently asked to explain its defense of certain people or groups — particularly controversial and unpopular entities such as the Ku Klux Klan, the Nation of Islam, and the National Socialist Party of America. We do not defend them because we agree with them. Rather we defend their right to free expression and free assembly.
Historically, the people whose opinions are the most controversial or extreme are the people whose rights are most often threatened. Once the government has the power to violate one person’s rights, it can use that power against everyone. We work to stop the erosion of civil liberties before it’s too late." https://www.aclu.org/about-aclu
Which is somewhat different than the SPLC:
"The SPLC is a catalyst for racial justice in the South and beyond, working in partnership with communities to dismantle white supremacy, strengthen intersectional movements, and advance the human rights of all people." https://www.splcenter.org/about