How do lawyers send information along with or after you receive your summons in the mail?

If you get a ticket or an underage or a dui,sometimes (actually, most times) there will be supplemental private law firms sending their information and letters of intent to help. How do they do this? Also, how do you get it to stop?How do lawyers send information along with or after you receive your summons in the mail?They have people who read police reports to see who might need a lawyer. There's really no way to stop it other than to keep out of trouble and therefore out of police reports.