Telling the truth, no matter what the consequences are, is never out of fashion. The Marine Corps for its part doesn’t have to defend: just tell the truth in the right context.
First, it isn’t an official logo. So the Marine Corps didn’t adopt it. The sniper community did; and maintained it for a lengthy period through multiple presidents and commandants.
2nd, there is no evidence that the common usage of the SS in every single STA platoon (all 18 of them), over (at least) two decades, tied it to the Nazis. So for someone to get an a$$ burn over it: that’s on them.
3rd, the Waffen SS is just that: history. The men that pull these triggers are not historians. They study military history in the context of tactics, weaponry, and battles, not of uniforms that have no consequence to the present.
4th, to suggest that it was “mind-boggling stupid” just impuned every single battalion commander, battalion Sergeant Major, battalion staff, inspector generals, AND every single Marine, sailor that has served within STA commmunity for 20 years. That is A LOT of people. Really smart and talented people. A competing hypothesis is: perhaps there is nothing there (in terms of intent to use the symbol) because the users of the symbol never could have anticipated a society so hypersensitive about irrelevant issues, and so focused on their collective vanity.
Lastly, if folks are going to get uprooted by a symbol that has no connection to our current military (as the evidence suggests), perhaps they need to spend more time in being offended by the constant erosion of the U.S. Constitution.
Symbolism doesn’t matter to you? History doesn’t matter? You say that these soldiers are not historians, which is I’m sure factual, but how can they be so ignorant of recent (20th Century) history? I guess you wouldn’t care if they wore swastika armbands, or a single red star on their hats...