It is not currently being reclassified, it is "under consideration" - that means they may study it and redefine based on dosage, or completely reclassify as a drug and remove from the supplement market.
As to "Why" - it is a periodic review - as more substances become promoted as (unregulated) supplements, the FDA looks at them with an eye to whether they are safe to be used in that manner. The long-standing protocol has been "once a drug, always a drug" - sometimes they prevail, sometimes the supplement manufacturers do.
As for dosage, I am not a pharmacologist, but I know the "detox" dose is very high and can be quite risky, but I don't have numbers for you.
Sue