I don't like the idea of my browser trying to protect me from social problems. I'm fine with my browser warning me that a site may be a risk to my machine, but I wouldn't trust my browser to display content to me reliably if my vendor was using it as a platform for any activism that isn't narrowly focused on keeping web standards open.

Perhaps I'd tolerate it as an opt-in, but even then, I'd be leary and start looking to switch because I think it's a bad precident to set, and (slippery slope warning) could go toward normalizing similar warnings for other issues.

If there's going to be a warning, I think it ought to be produced server-side.