When you insert the table in the sample XHTML file, there is no <thead>, and the <table> tag is visible. If you view table markup and delete the <tbody> tags, it is apparently still valid, since it does not fail a completeness check. The <table> tags are also still visible. However, you can no longer see the grid if you turn off table markup. It remains in "view table markup" mode.
What kind of rendering do you get in a browser with <table...><tr>...</tr></table>
I guess maybe IE and others are pretty forgiving and go ahead and render it as a table, without the <tbody>?
In any case, I can't do anything to get the <table> tags to disappear in my version 5.4 M050 using the standard sample XHTML document.