Why is it a custom in Baptist Churches to vote on new congregation members?

My friend recently became a member of the local Baptist church. When she and her mother wanted to join, they went to the preacher and he told them there would be a vote among the congregation members to determine whether they would be accepted into the church. She and her mother had to stand before all the members, and with a show of hands, the congregation agreed to their joining the church. It struck me as odd because it never occurred to me that there was an acceptance process for the Baptist church. Is this true of all Baptist churches? And what is the scriptural basis for this process?