R.
1. Since placebos are nothing but harmless, inert substances (like
sugar) you most certainly can buy them over the counter. But the point
is that you don't know they're placebos.
2. The placebo effect works, but is not entirely understood, and thus
is not a reliable method of disease control. Selling something
unreliable as if it was reliable (not to mention selling cheap, inert
substances as if they were expensive chemicals) is fraud.
So basically you're asking why fraud isn't legal.