Marleen Matlins character

Somebody please explain. I have two questions

1) Her character was able to lip read most people in the movie, but yet when she was interacting with the black cop she needed a sign translator?? The cop was talking normally like the other people so why couldnt she read him?

2) Why is it sometimes she elects to use sign language to communicate and other times she talks. Why cant she just talk for the whole movie? Is this because to remind the audience she is deaf?