The spellchecker in SBS works in a different way - when you click the button it sends one copy of the message to the server and parses it for errors, so there is no concept of it "staying on". It's like a submit button - it does not work as-you-type, as that would result in too much network traffic. All the depressed status of the toolbar button means is whether the underscores loaded from the previous click are visible. If you type more stuff it's not checked.
Browser extensions can parse in real time because the data is not being sent anywhere.