The t-shirts are in protest to government corruption. Nearly $700 million had been deposited into the prime minister's personal bank account. No explanation of who or why, just his assurance that there's been no wrongdoing.
People being dissatisfied with this and expressing that dissatisfaction by wearing protest t-shirts is being called a national security threat. More like job security threat (for the politicians), but it's not as if they're suppressing free speech:
“The reality in Malaysia is that there is freedom of expression without fear of prosecution,” he said, speaking on the condition of anonymity.