In addition to Rene's answer, it must be understood that the girls willingly participate in this activity. None were coerced. They are intrigued by the mysticism of Tituba's Caribbean occultism; they feel oppressed by the stricture of their Purtian upbringing.
Furthermore, Purtians believed that behavior such as this would surely paint both the participants and their family as doomed, and not among the "elect" (those pre-destined for salvation.) The girls' illicit activities in the woods, therefore, had ramifications not only for their own souls, but the salvation of their family members. This is one of the many reasons Abigail becomes so intent on keeping their "sin" a secret.