why did hughes title the story, thank you maam?

A grateful and now respectful Roger wants to say more than his mere "thank you" to Mrs. Luella Bates Washington Jones as she closes the door after taking him to her home and feeding him, but he does not have time. This phrase of "thank you m'am" is used by Langston Hughes as the title, perhaps, because it expresses the gratitude Roger feels toward this kind and generous woman who has effected a change in him.
After all, it is thanks to Mrs. Jones that Roger has not been arrested for stealing her purse; it is thanks to her that he has cleaned his face and had a hot meal. And it is thanks to her that Roger has been taught how wrong it is to steal from others as he is reprimanded physically and verbally by Mrs. Jones. Roger has felt the love of a woman with a large heart who has recognized a neglected boy in need of love and direction. The generous Mrs. Jones has given all these things to Roger. As she sends Roger on his way, he turns to thank her for her kindness and for disciplining him. But he feels awkward and embarrassed in his shame for his earlier actions toward this big-hearted woman. 

The boy wanted to say something else other than “Thank you, m’am” to Mrs. Luella Bates Washington Jones, but he couldn’t do so as he turned at the barren stoop and looked back at the large woman in the door.

As Roger departs, the reader has the sense that Mrs. Jones has brought about a change in this young man.