Is there a reason WHY they do this?
Very simple - they became Christian through the contact with Protestants so they accepted the tenets of American Protestantism with some peculiarities of their own (each of numerous Protestant churches has some specific beliefs or practices which puts it apart from others).
I did some search and it appears that they were founded by Moishe Rosen who converted to Baptism. So this would explain the similarities to the main stream Baptism.