Unlike in the "Land of the Free," the Thai government isn't trying to do this secretly. "The Technology Crime Suppression Division (TCSD) recently announced that it plans to 'monitor' conversations on LINE as well as other social media sites such as Facebook, YouTube, and WhatsApp to remove 'threats' to the national security."
The same reason is given as in the US, national security, but with no prevarication about what they intend to do.
When I was there in June, the Line was a hot new messaging system that Paula's friends were using. Her phone didn't have the app, but others with newer phones were excited about it. We looked at some new phones and she only wanted one that included the Line.
And now the commander of the Technology Crime Suppression Division has said that they already monitor the Internet for "messages with words that pose threats to national security, such as coup, monarchy, lese majeste, drugs, counterfeit goods and prostitution.”