Do you know if that holds true for other Eastern European countries?
I suspect it holds true but would have to do some research in order to make my answer anything better than an annoyed guess. I'll try to look it up over the weekend, but don't promise anything.
Could it be because of the import taxes in Poland?
Maybe? Same as for the first question. In the meantime: here's the best reference database compiled by Polish government that I could find.
And yes, it's pronounced exactly like Ishtar. The contrivance of the acronym and its full form suggests it was a deliberate choice.