If two (or more) random people are just talking, exchanging information about "How to grow weed", that is all "Theorhetical" and as such it is protected by our "Freedom of Speech" laws.
On the other hand, if a Hydro Shop is actually selling equiptment, or providing information that would - "Knowingly" - be used to grow (an illegal substance), that would be "Aiding and Abeting" in the commision of a crime. The shopkeepers are forced into a tight spot because it is unlawful to profit from an illegal "operation" and it is unlawful to sell things (or provide professional information) that will, knowingly, be used for illegal purposes.