Scientology is indeed a textbook example of a "Weird Cult".
L. Ron Hubbard decided to take his science fiction writing to a new experimental level and he created his own religion. The Church of Scientology. He proposed a new 'science of mind' he dubbed "Dianetics". The whole thing sounds superficially plausible, but upon scrutiny is found to be unaccompanied with any verifiable evidence and is inherently unscientific. But this problem has never stopped religions from flourishing.
Nobody decides to be a member of a cult. The word 'cult' has negative connotations of brainwashing or manipulation. Cult members usually report not feeling manipulated, but they are often sheltered from mainstream thought that might expose the cult.
I don't know about Cruise's or Travolta's reasons for affiliating themselves with Scientology. People become attached to religions for all sorts of indefensible reasons. It may seem completely logical and even personally beneficial when viewed from within one's religious circle. Only when viewed with a skeptical outside perspective can we objectively shed light on where these concocted religions stray from reality.