Seems to be VM being cautious (perhaps over-cautious) about licensing issues.
I think they say that they only support TVA on tablets through apps - not through a browser. I'm not sure I understand the logic behind that. And, of course, they don't have an app version for Windows.
VM have not made an app for windows, but for some odd reason they dont support it through the browser on such devices. Pretty stupid though cutting out a section of their customers when they could no should allow you to use it through a browser.