A computer program is a tool. If I toss a hammer off a scaffold and it hits someone, I'm responsible. I can't just say "the hammer did it, not me." The crime or non-crime I'm responsible for may vary depending on the circumstances. If I threw the hammer on purpose or accidentally kicked it off, whether I took sufficient safety precautions, etc.
These artists are clearly responsible for whatever their program did, and it purchased illegal drugs from a website. If Switzerland doesn't have a legal distinction between purchasing drugs and possession or use then hopefully smart cops, district attorneys, judges will take the mitigating circumstances into account. But it doesn't absolve the artists of responsibility.
Incidentally, where I live (and I believe in the US, probably in Europe as well) you can absolutely be held responsible for the actions of your children, until they reach an age where they are legally determined to be responsible themselves.