To my knowledge, they aren't in any way Protestant (in the Protestant Reformation since of the word).
Isn't not having a bishop Protestant?
No. A lot of Protestant groups have bishops.
Right. Lutherans, Anglicans and Calvinists all used to have bishops. Anglicans still do, and some Lutherans. Most Calvinists later took a presbyterian polity (though some went into congregational-styled groups), but a few remain in Anglican circles. Methodists also still have bishops, though their polity isn't traditionally episcopal these days. Before they joined with the United Brethren, the biggest Methodist denomination in the US was known as the "Methodist Episcopal Church."
Lacking an episcopacy is a hallmark more of the Radical Reformation, of which Baptists, Amish, Shakers, Quakers and Mennonites are the modern descendents. Though some of these employ an episcopal polity (such as the Evangelical Baptist Church of Georgia, which was mentioned above). Likewise, some denominations have split off to become non-episcopal (such as the Holiness and Pentecostal churches that come from the Methodists...though the African-American Pentecostals have a rather traditionally-structured episcopacy, more-so than the Methodists of today).
When I say "Protestant" I'm more referring to the ideas of Sola Scriptura, Faith Alone by Grace Alone and other, related concepts that are alien to the historic Christian Faith.
