I know some people where English is their second language prefer to have someone with him with a better understanding of the language when talking to a doctor or dentist to make sure things are understood.
My aforementioned Cuban friends some of them go with their parents to the doctor even though they do speak English they feel better about getting all the information right with my friends there.
I also used to work for a call center dealing with insurance benefits. I was friends with the spanish interpreter in our group. She said it's not uncommon for the people she helps to know English but feel better working with a translator to make sure something this important is done right.
I know if I was in Germany for example even if I spoke German as a second language and I was at a doctors office I wouldn't mind someone fluent in both with me to make sure everything got communicated properly.
I agree with your point. I would want the same. Would you expect the law to require the doctor to pay for the interpreter? I certainly wouldn't.