(eg. EE is immune to all retreats, so a HG's pushback doesn't work)
Oh - that brings up an interesting question.
I disagree with the above statement, because a pushback is not the same as a retreat. To me, a "pushback" means the Hill Giant hits something so hard that it goes flying; a "retreat" means that the unit runs away on it's own. The Earth Elemental doesn't get scared, and so it never runs away... but if you hit it hard enough then the unit will still end up in a different hex.
(Of course, you could be right. I'd be interested to see what Days of Wonder says in their FAQ.)
I completely shared your opinion for exact the same reasons ... until I found the following official statement in the creature compendium (#8 in the HG entry) on the DOW BL-page:
Hill Giant vs. Earth Elemental (Comment / Question)
Question: This came up in a game I was playing with my son. My Hill Giant attacked his Earth Elemental, which I actually had completely surrounded (all 6 hexes). I rolled two lore. Does the Earth Elemental retreat or not? Hill Giant says must retreat, Earth Elemental says never retreat. Which happens? Thanks very much.
Answer; Elementals such as the Earth Elemental never, ever retreat. In this case, the Earth Elemental overrides the Hill Giant's push back, and ignores the retreat.