From the petitions.whitehouse.gov petition kindly submitted for comment here:
"The United States is one of the few countries left in the world who still have not converted to using the Metric System as a standardized system of measurement. Instead of going along with what the rest of the world uses, we stubbornly still adhere to using the imprecise Imperial Unit - despite the fact that practically every other country that we interact with uses Metric."
This petition has the same problem most petitions submitted to the White House have--its factual premise is incorrect. I'm an American who has lived in another country (Taiwan) for years. The National Institute of Standards and Technology reports that "The United States is now the only industrialized country in the world that does not use the metric system as its predominant system of measurement."
http://www.nist.gov/pml/wmd/metric/upload/1136a.pdf
But the same government report notes that
". . . . In 1866, Congress authorized the use of the metric system in this country and supplied each state with a set of standard metric weights and measures.
"In 1875, the United States solidified its commitment to the development of the internationally recognized metric system by becoming one of the original seventeen signatory nations to the Treaty of the Meter."
In other words, the United States has treated the Metric System as official and legal since before my great-grandfather was born. The customary measurement system is, by contrast, simply customary, not mandatory. The United States has been metric since 1866 "in the sense that Americans have been free since that time to use the metric system as much as they like."
http://www.unc.edu/~rowlett/units/usmetric.html
If a particular individual or corporation engaged in manufacturing or trade wants to use metric measurements to meet customer needs and gain a profit, no one in the United States is stopping that. If someone desires to use customary measurements out of sheer habit from long-established custom, no one is stopping that either. My late dad the industrial engineer was aware of plenty of industries in the United States that from the 1970s, at the latest, had gone fully metric simply because those industries were involved in vigorous international trade. Perhaps the best governmental nudge that can be given for more use of metric measurements in the United States is more encouragement of developing international markets for domestic businesses.
I note that customary measurements are often used in other countries even long after metric measurements are adopted officially. For example, the Republic of China (the regime that governs Taiwan at present) has been officially metric since before I was born. Japan (the former governing country in Taiwan) was metric from the 1920s. But the unit of weight for vegetables bought in an open-air market in Taiwan is still the traditional 斤 (jin "catty," or Chinese pound), although that is now standardized at 500g. Prices are given in monetary units per 斤 for most fruits and vegetables to this day in markets in Taiwan (and in China).
A Facebook friend with a scientific education recently told me about the saying "A pint's a pound, the whole world round." If the United States begins using metric-standard units more for selling foods and the like, then perhaps a half-kilogram (500g) package will be considered to be one new "pound," just as a half-liter (500ml) package will be considered to be one new "pint." It is interesting to me that traditional Chinese culture and traditional British culture both had weight units in that range, about a half kilogram even before standardization to metric units. How are grocery measurements treated in Britain these days?