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.
True, it's either misinformation or lack of knowledge about the subject. but the thing is most of these people calling bitcoin a scam are not really interested in bitcoin and even if you try to
explain it properly to them they will simply ignore whatever you tell them and will stick to what they believe. but I don't also blame them since bitcoin has been associated with different
kind of illegal activities including scams.