What are you talking about with the weed? It's sold in pounds, ounces, quarter ounces and "half quarters" which is as ridiculously un-metric as it gets.
My theory on this is we use the imperial weights when describing the size in relation to the jail time associated with getting caught with it and then switch to metric for personal use.
And all Imperial amounts are based on the SI system (the official name for metric). That's basically the point of having a standard.
I'm really not sure what the big deal is that the packaging uses the metric equivalent of the unit used in the US, which is one of the main countries that many businesses worldwide serve. That's also the case with butter (1 lb to 454g), cream cheese (8 oz to 250g), etc.