It got so bad they ran a promo that if your chips weren't hot and fresh they'd give you a new batch for free.
Guess it cost them too much because they killed that promo pretty quickly.
However the reason the salt sticks nicely is it's applied straight after the fries are pulled from the fryer so they are still wet with oil.
When you order no salt, they wait until the fries cool down for safety, and adding salt once the oil has dried off just causes the salt to fall to the bottom of the container.
Also, it's been known for decades that you ask for fries with no salt so they have to make a new batch as they salt them immediately after cooking.