Usually they simply just don't have enough information or get the wrong information from unreliable sources. They don't understand something, but that doesn't make it a scam.
If they do not make money immediately then it must be a scam. People can be crazy sometimes with their beliefs and way they make excuses just so they do not blame themselves.
How can someone get involved in something they have such a lack of understanding?
I'll ignore them, they aren' worthy of your time and effort because these type of people are close minded and no matter how good you educate them, they won't accept it, it's not my lost anyway its there's. For me, what matters most is that I know bitcoin isn't a scam and if people will insist it's a scam then let them be, i'll do my thing and I know they will only swallow there words and realize that bitcoin is 100% legit and very far from scam.
Their opinion doesn't matter to us. Let them believe as they wish and live their life and we live ours. Out chance of financial freedom is much closer than theirs.