I strongly suspect that since many people don't like whatever conclusions can be infered from trolley problems, they try to dismiss trolley problems as "dangerous". If I find something really dangerous, it is the willingness to label uncomfortable ideas as dangerous when there are no better arguments around. The historical set of "dangerous" ideas includes heliocentrism, evolution, atheism, legal homosexuality.

Actually, nobody has yet demonstrated that in reality, people who are used to think about trolley problems or other simplified thought experiments are more prone to bad thinking.