Because it will deter a disproportionate number of people. This isn't a binary situation. Proportionality matters. If the World Health Organization issued a

Because it will deter a disproportionate number of people. This isn't a binary situation. Proportionality matters. 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May 2016 will have an increased risk of death from bees, you might be annoyed to have to dig through obscure, possibly unpublished analyses to find that the increased risk was +0.001% due to projected shifts in the bee population relative to 2015.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.