I think in general adults should be able to make up their minds about ingesting chemicals that might hurt them. However, it makes sense to make an exception -- if the chemical has addictive properties and empirically is likely to have network effects (i.e. in practice it tends to spread through the community), it seems reasonable for the government to step in and regulate the substance in more substantial ways than enforcing disclosure. I'd argue adults don't make choices with respect to these chemicals out of their own free will, but are coerced into consuming them through biological/social rewards mechanisms, and I think it's the government's responsibility to step in.

For example, regulating psilocybin seems like an obviously bad idea (it doesn't seem to be addicting and doesn't devastate communities), where as regulating crystal meth is an obviously good idea.

I don't know where the line is (e.g. what about cocaine? it's addictive and has propensity to spread, but isn't nearly as bad as meth), but that doesn't mean there aren't obvious extremes. I also don't know what the government can do about it (making distribution illegal doesn't seem to work), but philosophically, I don't think it's unreasonable for the government to step in if it can be effective.