Cannabis when grown responsibly and naturally and when used responsibly by adults is near harmless and has many benefits.
When I use knives in my kitchen; I know they're sharp but I use them to do LOADS of useful things such as cut up vegetables to make food for myself and my family and friends, cut plasticy labels off things, open packages in the post. However I would never give children knives or cannabis to play with.
You COULD outlaw ALL knives for everyone as they are used to kill and harm but then, people would realise they quite like knives and their many benefits and so a trade in black market knives would thrive and people would be looked down upon for using them when others get by quite happily without them. An illegal market full of hardened criminal would take hold of the knife market and like hardened criminals do, they would tarnish the name of everyday kitchen knives with those of hunting knifes, machetes, daggers, and even harder weapons like guns.
Then people would go around ignorantly saying "oh but kitchen knives are DANGEROUS, they ruin LIVES, they break apart families." or "My mate started on kitchen knives and now he's not happy without an AK47." or "Kitchen knives are ILLEGAL for a REASON" without ever really THINKING about WHY they are illegal, without taking into consideration that the MAJORITY of people in ALL walks of life, from doctors and nurses, to lawyers, firemen, teachers, mums, dads, grandparents, plumbers, shopkeepers, electricians, IT support, consultants, recruitment agents and pretty much every type of person on the planet actually use knives, enjoy using knives, enjoy knives responsibly, while holding down a job, a mortgage, excelling at life in multiple ways, having happy and fulfilling relationships with their families and friends and feeling that the benefits of kitchen knives not only FAR outweigh the negatives which are exaggerated in the press to instil fear in the ignorant (which is very annoying as almost EVERY journalist is guaranteed to have indulged in some form of knife themselves) but also that using knives is well worth the risk of criminalisation as even most of the police understand that kitchen knives are not like other knives and they really aren't going to be bothered to go around chasing people who just want to use kitchen knives as the law is according to the scientists paid by the government, a very very stupid one. Also, if knives make music sound this good... everyone should at least try them.
Sorry for getting carried away in my metaphor