Because the garbage heap is exactly what where those stored embryos will go when their "parents" stop paying storage fees to keep them.

We aren't talking about embryo farming here – and we would oppose that as well – but we are talking about stem cell material that exists and is simply going to be destroyed unless it is used for medical research.

To do this is no more immoral than a parent who donates the remains of their dying child to science for medical research. The parent hopes that by doing so, someone else will be saved death or suffering. The exact same thing is true in this case.