In the US we call them "assistance dogs" or "service dogs." Under Federal law they are allowed to go anywhere (unlike Britain, "regular" dogs can't travel on trains or local busses in the US, and health regulations generally prohibit allowing them in places that serve food). Under the disability law it is discriminatory to demand to know what service the dog performs, (not always obvious if the dog is trained to assist with a hidden condition like epilepsy) and there is no official process of certification for the dogs, so in theory anyone can claim their dog is an "assistance dog." I haven't heard of any cases of abuse, however (but most people are unaware of the provisions of the law, so that could change).