In some areas, teens involved in "sexting" are being prosecuted as purveyors of child pronography (in some cases, teens have been prosecuted for sending pictures of themselves nude - see "Legal Cases" on the Wikipedia article about Sexting). That's probably why elders are required to contact the Legal Dept. if teens are found to be involved with this activity. With so much uproar among the "worldly" media, it was inevitable that the Society would feel the need to take a strong stand on the issue.
Was this letter read to the congregation, or was it a confidential Body of Elders letter?