THE GOVERNMENT IS ALREADY INVOLVED.
The fact it's illegal means the government is involved. What you're arguing for is continued GOVERNMENT INVOLVEMENT.
It's no more reasonable to suggest that legalizing weed is going to lead to sugar being monitored and test as it is to suggest that keeping the government involved with making weed illegal will lead to sugar being illegaled.
You're seriously trying to argue for continued, tangable, factual, UNQUESTIONED government involvement by putting forward a boogeyman of hypothetical, questionable, unknown government involvement.