This is just a tactic to keep the general public constantly afraid of the secret boogeyman hiding behind every corner just waiting to strike at any minute. They even go to say in the article this:

“We have no intelligence indicating that there is a specific threat to Boston or the Fourth of July celebration that we have here

Well if you have no specific information about an attack why are you warning people? That's like me letting everyone know that I've got a really good chance of winning the lottery but I don't have any inside information, some magic ticket and my odds are the same as everyone else who bought tickets in the same round.