I just tried to test on Windows Phone 7.8 to pin to the start screen, but it just does the default "snapshot" of the website instead of the live tile. I'd be interested to see what behavior occurs on Windows Phone 8.0. My theory is since Windows Phone 8.1 and Windows 8.1 share a more similar core than before is why the tile works..
(upon further research
):
http://www.winbeta.org/news/windows-phone-81-lets-you-pin-website-y...
So only Windows Phone 8.1 or higher supports it I guess... Oh well.