You're definitely right. I think when you try to rationalize it though, the more sophisticated argument is that in the soap example, you're creeped out because you feel like you're using a person as a means and not an end in themselves. It feels like it is disgracing someone you never met, that was physically transformed by cruel inhuman processes, which in turn disgraces yourself.

In the instance of cremation, you are saying goodbye to a person and not using them as a means to an end, it is a form of respect. The individual isn't commodified on a practical level (though the funeral industry has something to say about that).