The prior attorney is correct. Once you are arrested, your arrest becomes a matter of the public record. Attorneys then pay people to scour the public record to glean the names and addresses of people who have just been arrested.
Many of my clients have described getting tons of embarrassing mail from these attorneys who hope that you will hire them to represent you.
You are in the right place and I would find an attorney via Avvo or another site before I would hire an attorney I learned about through one of these letters. In my mind, they are the modern day equivalent of "ambulance chasers".
Arrests are a matter of public record. Many fine attorneys send out letters in hope of attracting new business. Check out any potential lawyer through the state bar association.
My answers are intended only as general legal advice and are not intended to create an attorney-client relationship. There is no substitute for a full consultation with a local experienced criminal defense attorney. For more answers based on my 19 years of experience visit my website, www.austincriminaldefenseattorney.com