The church members believe it. Some like myself believe it because they believe God has promised to preserve his word to all generations, and we believe the KJB is that preserved word in English. Others probably believe because that is what they have been taught.
I won't use any other version. When I got saved 50 years ago, at first I had several different versions and that caused me to be confused. That is why I studied the subject. I came to believe the KJB is the preserved word of God in English.
No, lots of folks come to our church with other versions, nobody says a word to them. They can use whatever they want. Now, if they wanted to get up and preach from another version, that might cause some heads to turn.
One thing that concerns me is that those that are not kjv only will condemn those that choose to be so. What is written above, I don't have a problem with. I don't agree with it, but there's nothing wrong with a church wanting to only use the KJV. Obviously I don't agree with Winman on his first paragraph here, but I do have respect for a church that wouldn't condemn someone if they choose to read from another version. My church actually used the KJV. I read the ESV, but use the KJV when I teach. (yay for my ipad!)
As for the OP, no, KJVO(like what Winman has posted above) shouldn't be called a cult. There are some(Ruckman) that are more like a cult.
Generally speaking, I would say they are misguided and in error. Some KJVO churches are antagonistic towards those they consider to be in error. Case in point: there is a church on a back road to Annapolis, MD that has sign on their property that reads: "We only use the 1611 Authorized Version". What they are actually communicating to the public is, "We use the KJV, and if you do not, you are not welcome."
My though whenever I see a 1611 sign is, no you don't.
sometimes I wish they would read the 1611 are more importantly the margin notes and the preface. A lot of dumb ideas would go away quickly.