It saddens me to see it when people fall into those traps. I think the reason it happens like that is because some people are just desperate for something, anything, to change the situations they're in. Many are greedy themselves and go there to "sow seed money" because these preachers promise them that if they do, God will give them great wealth or make miraculous things happen. Some are sick and want to be well so bad that they'll do anything for it.
It's very easy for desperate people from all walks of life to believe something that may help them. For those that believe in God, it's easy to suckered, to be honest, if they're not totally familiar with what the bible says the nature of God is.