I have to disclaim I don't know what meth is like, what meth users are like, or what it's like to be on meth, but if 20 people are doing meth and not bothering anyone, and the police officer is keeping watch to ensure they don't bother anyone, that is fine by me. We should treat them like drunk alcoholics, not like murderers (unless they murdered someone).
Separately, we could encourage people to try less harmful drugs than meth. That isn't really possible as long as we give out the same penalties for weed as for meth.