That's one of the cultural differences between the UK and the USA. In the USA all sausages I have seen are pre-cooked and just need heating. In the UK all sausages are made from raw meat and need thorough cooking before eating.
That’s utter nonsense, sorry.
As an American who’s lived in USA and continental Europe in roughly equal amounts, and who visits the UK regularly... nope. All of these places have sausages that are sold cooked (hot dogs, frankfurters, wieners, kielbasa, etc) and sausages that are sold fresh (italian sausage, bratwurst, breakfast sausage, “bangers”, etc).
You’re in USA, have you never seen fresh brats, italian sausage, chorizo, breakfast sausage (links or bulk), etc. in the meat case?!? When I lived in USA, I practically always had hot italian in the fridge.