Tex is not rendered in the preview and unfortunately there is not much that can be done about it - preview is fetched from the server without reloading the page, and MathJax script runs on a page load. On the other hand SMILES images are created server side, so they are displayed in the preview.
We can devote some thread to testing and delete the content now and then, no problem. I can't think of a better solution at the moment. But I am open to suggestions.
It is possible to create some simple sandbox for MathJax, but it would be not integrated with the forum itself, not the idea that I like.