Most of my sewing and measuring things include both metric and English measurements. And we don't really even use the English system, I assumed that was common. It is handy when using American recipes.
A great many of ours do, too--and it's useful when using European recipes.
But the smaller measuring spoons and the dry measuring cups are one-system only.
And all our recipes are written that way (as you point out).
I remember once finding an op-ed piece of some sort written by someone who advocated the "use the scale" method of measurement. He said that *it was a conspiracy* to keep the recipes in magazines, cookbooks, etc., in the imperial system.
Since at the time I worked for a high-profile publisher of food magazines and cookbooks, I got a huge kick out of it.
My biggest problem is when recipes (regardless of nationality) call for one can of this, one cube of that or one package of something else... without saying how much is in a can, a cube or a package!!!It can be equally frustrating when it's done here, too. I tried to help a friend redact an old family recipe from her DH's late grandmother. "Take a 50-cent box of vanilla wafers, mush them up, put them in the blue bowl and add milk until it looks right."
At that same publishing company, they wanted to drop the "1/2 cup" and just start saying, "1 stick of butter." I guess for now everybody would know, but it just really bothered me, as a copyeditor.
"Stick" is not a universally agreed-upon unit of measurement.
My mother tells the story of her best friend's mom, who made the most wonderful biscuits. Until the coffee cup she was using to measure broke, and the new one she got was a different size and held a different volume of flour.
And I once read a mystery where the heroine ran a bakery, and there were recipes sprinkled throughout the book. She measured her flour by PACKING IT into the cup very firmly. I thought that would probably be a more accurate way to measure flour by volume than sifting it is.
I had a never-fail choc.chip cookie recipe that suddenly started going wonky. I couldn't figure it out. I theorized that I was not getting the right amount of flour, so I started measuring, then weighing what I was measuring. I discovered that actually it was pretty consistent. (Turned out to be that I'd forgotten the 30 seconds that belonged on the cooking time--I had a timer that would keep the minutes for the next batch, but lose the seconds.)