More likely is that the default COMCODE_PAGE_SCREEN.tpl is coded to not show it on the start or rules pages, just because those are typically more design-heavy than content-heavy.
I was thinking the block showed on the forum index too, not sure why. The Google+ link doesn't show for Guests though, shows when I login, but I've not entered any Google info to my profile.
There's nothing coded against that button for guests, and I'm seeing it in incognito windows when not logged in (i.e. guest both on Google and Composr)
More likely is that the default COMCODE_PAGE_SCREEN.tpl is coded to not show it on the start or rules pages, just because those are typically more design-heavy than content-heavy.