Because icebergs ARE made of fresh water. Don't believe me? Mix up some water and a lot of salt and just TRY to make an ice cube out of salty water. You can't. When water freezes it leaves behind almost all of its contaminants (dissolved and particulate) in the remaining liquid. Very picky that ice is! read more
They are of course made of fresh water, Iceberg are part of the Ice cap that was formed from snow. There is no way to get much salt in there. Even freezing very slowly salty water will tend to separate salt from the freezing water, This behaviour used to be utilised even in laboratories and know as zone crystallisation. read more
Icebergs form as a result of two main processes, producing a freshwater iceberg: Ice that forms from freezing seawater typically freezes slowly enough that it forms crystalline water (ice), which does not have room for salt inclusions. These ice floes are not truly icebergs, but they can be extremely large chunks of ice. read more