Satanism basically takes a world view that God is evil and Satan is good.
In their childhood they were treated poorly by people claiming to act on behalf of God to the point of God being blasphemed among the heathen because those people.
Since this was their primary experience, you will find people being kind to others and a satanist. This is because the God of the church is associated with evil in their minds. This association is created over years of the followers of God behaving in an evil fashion towards these children.
There are other forms of satanism, so it doesn't always apply. However, since the church has been associated with poor treatment of the LGBT community, the satanism merchandise makes sense.
When things like this happen, it is up to the church to respond in a more Jesus like manner in the future towards all people.