It is true that some of the hisec ice occurs in 0.0 as well.Ice is ice. There are different kinds, but the kind found in 0.0 is identical to the kind found in highsec.
However, 0.0 ice belts provide ice types which have a much higher yield than the ice in hisec. For example Krystallos can only be found in 0.0, Dark Glitter only occurs in a few lowsec systems and mostly in 0.0, and so on.