Agree, it's about possible reduction of harm. A notice that cars crash is not going to stop people from using cars. A notice that foreign travel is not safe will deter some people.
The problem is that foreign travel is safe and all this fear-mongering is unjustified.
99% of people who travel to a foreign country are completely safe. Saying anything to the contrary is pure fear-mongering.
If the World Health Organization issued a world-wide alert that going outside in the month of May 2016 will have an increased risk of death from bees, you might be annoyed to have to dig through obscure, possibly unpublished analyses to find that the increased risk was +0.001% due to projected shifts in the bee population relative to 2015.
That straw-man example of announcing nearly nothing would probably be in practice more statistically relevant to people's lives than this one. Yet, here we are discussing how "foreign travel is not safe" with no information we can use to determine how much anyone should actually care.
Which is silly when they're more likely to die in a crash on the way to the airport than during flight or while abroad. It's completely illogical and actually gives weight to the actions of terrorists.
If the gov't said "For the next week, car travel will have heightened risk", people might take notice.