You are correct about the US Marines making this decision. But I would almost bet that a big part of the decision was based upon the attitude of California lawmakers and peaceniks within the state. It wasn’t too long ago that returning troops were spit upon and called baby killers. Being a former career military person, I personally do not see anything wrong with making a quick stop for gas while in fatigues, BDUs, etc.
The Corps wants its Marines to appear in uniform, but in the appropriate uniform. Off base, the appropriate uniform is the informal Service "C" or "Charlies" (dress shoes, green trousers, khaki shirt and garrison cap) or the more formal Service "A" or "Alphas" (with belted green coat and khaki shirt and tie). Those are the minimums, and the Dress "C" (with blue trousers) and Dress Blue uniforms can also be worn when appropriate. Civvies are not the only option for off-base attire.
As far as I know, that has always been Marine Corps policy.
So if you see a fatigue/cammie/BDU/ACU uniform in public, it's a soldier not a Marine.
“But I would almost bet that a big part of the decision was based upon the attitude of California lawmakers and peaceniks within the state.”
In Oceanside, we hold parades for them!