Well, I dislike all kinds of invisible tracking, but that is not the point here.
WP and all the plugins listed in the directory are GPL licensed. I think it is only fair that a plugin that uses proprietary services to state so clearly.
My problem is this: site owners might the plugin and users login with their Facebook (or whatever) account. At no point does anyone see or intend that some other corporation gets data, too. It is dishonest to hide that fact.
The plugin's affiliation has since been made clear, so I have nothing to complain about anymore. Of course, they don't clearly say "We (can) track your visitors now", but they can probably not be expected to (them being a company). People are hopefully clever enough to infer that themselves. However, it is site visitors I am concerned about: do site owners tell their visitors/customers that by using the site, they can be tracked by a third party? Probably not.