It's because of the way the language is put forward. It says the crown has to prove that the substance entered the water. So what the crown has to prove is that the act occurred. At that point, it captures a group of people who are now guilty until they can prove their due diligence.
So it doesn't include the mens rea in the first part of what the crown is proving; it's only the actus reus. Then we get the reverse onus situation, and the result is that this group of people is now considered guilty until they can prove their innocence. That's our concern.