Acetylsalicylic acid is aspirin, it's a couple chemical steps away from what's in willow bark.
Anyway, my point is ---> in the real world, it's because of the chemicals in willow bark. But that it should likewise be true (or not) in a game setting should be a GM choice, not a basic assumption.
This is not a big difference from what you're saying. I just think you are saying that the base assumption should be assume all science is exactly the same as in the real world, and the GM can choose to deviate. I am saying the base assumptions should be only what is modeled by the rules, and science is something the GM can choose to inject into the mix if it suits them.