At the end of Act I, Tituba, Abigail, and Betty name it could be unclear why these people are named as those who have made compacts with the devil. The real reason that these people have been named can only be assumed to be because a few of them -- Goddy Good and Goody Osburn -- already have bad reputations and it would be believable that they would do something like this. It is important to remember that the girls and Tituba are completely lying about anyone every being seen with the devil. However, as the play goes on, people are named as accompanying the devil because having them (Goody Proctor, Goody Nurse, etc.) go to jail or be hanged will be beneficial to other characters in the play.
Miller's motive for this is the natural tendency of a child to blame someone who would be disliked within the family or the society at large. Even Tituba, pressured by Hale, decides that she must give in and so she begins by accusing Sarah Good, an outcast in the village, as the first "witch". Abigail takes up the accusations to bring the limelight from Tituba to herself and follows on with other unpopular Salem citizens. Betty continues the frenzy of confessions with the same mental justification--that these individuals are disliked anyway, and so thay can become scapegoats in order to free the accusers of guilt and punishment. This early accusation of outcasts becomes even more significant when respected members of the society are later accused.
In the beginning of the play, Tituba, Abigail Adams, and Betty all name Sarah Good and Goody Osburn as being associated with the devil. They do so, first and foremost for self preservation. Secondly, they specifically named Sarah Good and Goody Osburn for one main reason: they both had poor reputations amongst the town of Salem. An outcast and an alcoholic, respectively, are named by these three young females because it would not be regarded as unbelievable if these two women were accompanied by the devil. However, Abigail and Betty eventually begin to name certain people only for their own benefit. This, in reality not in the play, leads to Abigail's ultimate demise because she eventually names the wife of a high up judge which leads her to flee.