s:url tag usage in a public page triggers a warning to not expose JSP pages directly

I have a number of public pages that use the <s:a> tags with no issues. But one page uses an <s:url> tag, and every time it is visited I get a warning on our logs the Action invocation context is null, and that JSP pages should not be exposed directly. This is an informational page only, and I can't think why the URL tag is unsafe to use while the a tag is safe. I am assuming this is a bug, but of course if there is an issue with the URL tag on a public page I would like to know.