The Tea Party and American libertarianism in general has taken on the role of the anti-establishment conservative party. I think a lot of self described American libertarians are really just looking for a conservative alternative to the two main political parties. The problem is, they haven't changed their views to match the libertarians. They're still republicans, but they just don't like the republican branding anymore. By joining the libertarians they're making it a mainstream party but at the same time pulling it to the right on social issues.
Come November when all of the libertarians vote republican, it will not be because real libertarians care about economic liberty more than civil liberty. It will be because all of these libertarians voting republican are really just republicans voting republican.