SSL only on Main site in Multisite

I've got a multisite with pro-sites running on it with a standard SSL cert for the primary domain I've got it installed and running ok on the main domain, but the site is forcing SSL on all sub-sites. This is giving me browser privacy errors (understandably).
I've searched around but can't find a clear answer on how to set the site so only the main site has SSL and all sub-sites don't. It's probably something simple that I'm just missing....