Crowell does a good job of indicating the widespread popularity of the King. He doesn't explain very well that although Thailand (formerly Siam) is called a constitutional monarchy, elderly King Bhumibol has much more political clout than do the constitutional monarchs of Europe.
The "red shirts" are largely from the rural poor and the army's enlisted troops are their sons. The "yellow shirts" are from the urban upwardly mobile and the army's officers are their sons. As Crowell says:
Many conscripts hail from the same rural classes that dominate the red shirt movement, and it is clear that their officers are uncertain they would obey any future commands to suppress the movement.
When enlisted troops refuse to use their loaded weapons against insurgents who may well be their kin, they will likely turn those weapons upon their officers before changing sides. Class conflict within the ranks makes for very nervous officers when the enemy is domestic and the turmoil is armed class conflict.