I'm sure marijuana laws like that exist, but are you going to argue that they're actually reasonable? A law with exemptions for small amounts of weed sounds pretty stupid to me and serves better as an example of how politics can twist legislation into duplicitous shapes rather than an example of good scale-sensitive lawmaking.
How daft can you be to think that scale makes no difference? You can drink water and be hydrated or you can drink too much water and die. Scale is critical in determining appropriate rules.