Do cooks in the US measure volume using 'traditional' or 'legal' units?
Great question Chris!  I'm a US baker who has been baking for a LONG time and never gave it any thought whether my measuring utensils are "legal" or "traditional" - they say "1 cup", "1 Tsp", "1 TBSP", etc.  

In practice, all this has meant to me is that when my US recipes call for 1 cup of flour, I measure 1 level measuring cup of flour.  Same for teaspoons and tablespoons.  My European cookbooks refer to ingredients in weight (250 grams, for example).  In those cases, I would either convert those measurements into my more familiar cups, tsp's, TBSP's, etc., or use another set of utensils with measurements marked on the side.

While I've used the written measurements for a guideline, I've always also depended on the preparation instructions to determine the correct consistency of the dough or batter.  This is particularly true for flour and liquids.

Regarding uniformity in the measurement tools, unfortunately for the home cook, there is no one monopoly.  Every grocery store, dollar store and cooking store has an assortment of measuring cups and spoons.  Some of the better measuring cups, like those from Pyrex, have volumes written on the side of the cup to use as guides.