Call them con men, grifters, scammers or thieves. Or simply call them liars, because lying is what they do best. They stare you in the eyes, do it via email or over the phone, and lie through their teeth. They do it casually and with such conviction that we have no reason not to believe them.
Sometimes they call you or knock on your door trying to scam you. Whatever you tell them can be used against you. They can steal your identity. If they find out you don't have an alarm, they may break into your house. If you tell them the company your home alarm is with, they may call you at a later date posing as that alarm company and requesting "updated credit card numbers." They can also sell you a bogus alarm system.
The Detroit Free Press reports that scammers "come door-to-door selling free alarm systems or systems for $99. Then, they lock you into a long-term contract for three to five years. The equipment is inferior. I've known people that have been burglarized with this equipment, and the burglars just yank the alarm off the wall and it doesn't work."
This issue is best resolved by not answering any questions at all, hanging up, deleting the email or telling the person at the front door (while you speak through the locked door) you are not interested. No matter what, never give the scammer your Social Security or credit card number or reveal whether you have an alarm.
Only purchase alarm systems from reputable installers, and do your research to make sure the company has been properly reviewed and vetted for a quality product.