To keep the sheeple calm and controlled. Don't want the sheeple to get alarmed and start demanding. Don't want the sheeple to think they are in danger and start demanding. Don't want the sheeple to get too involved in what might be going on. Ignorance is bliss. Keep the sheeple ignorant, and they will remain fat and happy.
I'm convinced that's what's going on. It's the only explanation for this. People who are paying attention know perfectly well, or at least can read the signs, that we have been under attack by Islam for years, if not decades. But govts, not just our own, are unwilling to acknowledge this to their populace, for the reasons you just stated.
Then why warn people for weeks ahead of time that we are likely to see more terrorist acts, especally on July 4th? It still doesn't make sense. I can only conclude that political correctness is ingrained to such an extent that both government officials and media will grasp at any straw to avoid being accused of being anti-Arab or anti-Muslim.