Yes - you could have other translation problems if you liked. You could have recipes that called for 240ml of flour (although there are good reasons to go by weight); or you could have recipes that call of 3.5 oz. of flour. But right now we have a lot of recipes that are carefully designed to be easily measured under one system or another (75 g; 1/2 cup). Changing systems makes the measurements come out less conveniently, whichever way you choose to change them.

Of course, there are no shortage of recipes that do use the metric system, so basically this is a resistance to rotating out cookbooks.

You are correct about the terminology; technically, the system used in the U.S. is United States customary units. I have never heard that phrase spoken aloud, although this could be a problem with the people I speak to, rather than any indication of common usage.

[Edited to adjust for polymathwannabe's correction -- thank you. The 240 is what I intended, but the 250 was an error.]