If you must have a tab, it should have a big dismiss button for the 90% of visitors who are not interested. Why is there no option to close it?
Another option would be not to have a tab at all (a hovering tab is very annoying as it hides content and distracts from the site for a function which is not often used) - instead let customers include an unobtrusive help button in their actual page content wherever they wish which calls your js in order to display your overlay when clicked. You don't need an obnoxious tab.
Site owners actually have the option to make it close able, and implement with just a simple "help" button.
I think what generally happens is that the team that handles customer communication does not have the dev resources to get someone to add UI elements to their page, so only bigger companies take advantage of "closeable olark" and click to open the chat window.
We also found that once people close the window, many get confused and have a hard time opening it again. (clearly not a problem for the "average HN reader", who might clear cookies and reload)
Another option would be not to have a tab at all (a hovering tab is very annoying as it hides content and distracts from the site for a function which is not often used) - instead let customers include an unobtrusive help button in their actual page content wherever they wish which calls your js in order to display your overlay when clicked. You don't need an obnoxious tab.