I'm not expecting you to accept that philosophical position, but I hope it explains how the governmental policy is only an overreach to the extent that a particular unprovable and unfalsifiable philosophical position is correct.
There is a much brighter dividing line between gametes and embryos than between pre-viability and post-viability embryos, so just on the basis of legal certainty it would be better to say that life starts at conception. I also think that it is philosophically parsimonious to define human beings as unified entities with distinct DNA from their parents, otherwise each skin cell should have its own human rights (which would mean that obese people get to cast more votes than skinny people).
In any case, given that there is ambiguity on where to draw the line, the safest approach is to pick the earliest practical point, since mistakenly protecting non-humans is better than mistakenly manslaughtering entities that we later concede were humans. If we take the view that every gamete has to become a human life, then nearly everyone in the country should be arrested as soon as they reach puberty.