The problem with Superman is his gay morals.
Even though they are "undead" he would not use his heat vision, as he would see this as killing them.
Batman is actually able to differentiate, but Supes is just too much of a big gay boyscout, and would want to cure them.
You could also say that the heat vision is different from actual solar rays, as it has been metabolised by his body.
Just the same as solar panels are used to power electrical goods etc, its not actually solar energy that ends up running through your tv set to power it.
