Many Americans are quite happy with metric units. Unfortunately, they
are subject to mandatory constraints. In a free market, many more
products in the supermarket would be labeled just in liters and grams.
Manufacturers such as Proctor and Gamble have said that they would use
metric-only labels if it were not for Federal laws such as the FPLA
making it illegal. Such laws are called 'Technical Barriers to Trade'
because it makes it difficult for products to cross international
boundaries even if customers want to buy them.

In US aviation, the FAA controls the pace of metrication by its rules.
The mandated unit was Fahrenheit up until 1996, then it changed to
Celsius.