I think you've touched on one big source of this problem, which is that many of these icons have been trademarked and saddled with all manner of usage restricti

I think you've touched on one big source of this problem, which is that many of these icons have been trademarked and saddled with all manner of usage restrictions and licenses. As an entrepreneur or designer, am I going to waste who knows how much time interpreting the usage guidelines from Share This, or devote valuable space on my site to giving the Open Share people an attribution? No. I'm going to spend 5 minutes designing something that won't cause future legal threats or trouble at acquisition time. We need something with an MIT-like license on it.