Mostly out of ignorance many people abuse drugs, use them casually, without any respect for them or much knowledge either of how to use them or their dangers, believe in all sorts of myths about them, and even use them in self-destructive ways. Then they blame their negative experiences on the drugs.
It is possible to use psychedelic drugs constructively, in positive settings, and with knowledgeable sitters who can provide support when needed. Studies have shown time and again that such an approach maximizes the chance of very positive, profound experiences that are highly valued by the subjects for many years. That doesn't mean the experiences are risk-free. One could have a negative experience even when one has prepared thoroughly. But even negative experiences can be valuable if one faces them and tries to learn from them.
One other thing we should recognize is that people are going to alter their consciousness somehow. It seems to be a fundamental human need. The "just say no" approach does not work for a significant proportion of the population. Wagging fingers at that population and continuing to tell them "just say no" can even be counter-productive, as what is forbidden often becomes more alluring. Much more constructive and helpful is teaching people how to use these substances in a healthy, positive, and non-destructive way.