One was a military fighting unit including foreigners (but not limited to them, recruitment was very varied) that fought at the frontline, the other was mostly for internal „policing“ and holocaust organization (strictly volunteers and considered „elite“). Just because both are bad and share a similar name doesn’t mean you can use the names interchangeable.
Edit:
Why does this not apply to the SS?
Because they all volunteered and murder on an industrial scale was their purpose and official task. The Waffen-SA committed war crimes, too, but it was not their main purpose and on a wholly different scope.
I can muster some sympathies for i.e. a foreign prisoner of war that was conscripted into the Waffen-SS, or a foreigner who wanted to free his home and misguidedly sided with anyone who was willing to fight against Soviet occupation. I do not hold the same sympathies for Germans who applied as volunteers to join the regular SS because they really wanted to help exterminating Jews (and others) at home and in the occupied territories.