Actually, I'm not so sure - for example, I don't see fantasy attempting to do so in the same way or degree. Reading George R R Martin, for example, didn't leave me with questions about issues outside of the book - just questions about the characters and the world. And because it's fantasy, any questions about these belong only to the book and have no real relationship outside of the story.
I remember the issue raised on the GRRM board here about a major feature of the world was a massive wall of ice. Someone suggested it couldn't exist in the real world, to which the retort wasthat it was fantasy, and so it didn;t matter - in other words, what happens in fantasy doesn;t really need to have anything to do with the real world or our place in it.
I'd therefore suggestive that science fiction - by it's generally speculative nature - forces the reader to think about issues beyond the immediate story - because the questions they raise are as relevant in the real world, not least it's future, and therefore has a different inherent experience for the reader.
Anyway, there's a generalisation to be shot down to pieces.
EDIT: Just to add to poly's point - moral, ethical, socio-political questions can be raised in any genre, and cannot be the claim of science fiction - which by it's very nature surely has to raise questions far beyond these general literary stables.
2c.