Why most people don't know the difference between market research and telemarketing or soliciting?

I am doing telephone surveys and when I explain that I work for a market research firm, the majority of the respondents tell me that they are in the "Do not Call list" (which aplys ONLY to telemarketers and solicitors) or they say that they are not interested in buying anything, and they hang up, and they lose the chance of expressing their opinions about an specific subject and making their opinions count.