Rhubarb, I think the reason that people are assuming a mental illness on the mother's part is because statistically it's very likely to have been the cause of her actions. It doesn't follow, though, that we think that all women with a mental illness will do such a thing - absolutely not.
As others have said, the reason that men and women who kill their children are treated differently is that research shows that they have wildly differing motivations for doing so.
Men tend to have been controlling and/or abusive in their families, be seeking revenge on their partner, and choosing the form of punishment that will cause most damage.
Women tend to be suffering from long-term depression, to bear the full responsibility for the children with insufficient support, and to be searching for a way to avoid the future suffering of their child.
There was a good article on this within the last year (not BoF's link, something else) but I can't find it. I'll keep looking.
It's very upsetting, though. Really sad.