"Since they only have to throw in a few U.S. shrimp to call it that, is there a specific stamp or phrase we can look for in order to get only U.S.-caught shrimp?"
Not that I am aware of. The only way I know to assure that the shrimp are caught in U.S. waters and fresh is to buy directly from a fisherman.
"How about in restaurants, do they have to say where their shrimp is from?"
My daughter works for a resturaunt that specialises in po-boys and seafood. The owner says they get their seafood from a distributor and the packages are stamped "Gulf Shrimp". That's as much as she knows about the seafood she serves.
My husband has been out of this industry for several years because he was not as opptimistic as his cousin. Having our daughter brought my husband in shore. Tighter regulations, a storm that wiped out more than 70% of my husband's equipment then a major break down in a small time span forced my husband into a land job. A job loss and our current economy sent him back out there after 15 years of being on land.