1. Like you say, they never came out and said she was a prostitute.
2. John Ford had a lot of pull as a director.
3. Claire Trevor was (along with the rest of the cast) fantastic.
I’m guessing that since it wouldn’t be obvious to anyone under 18, unless extremely precocious, and it was only alluded to visually, and not really spelled out, it was let go. But there were a lot of movies that surprisingly passed, albeit with a higher age warning.