keen.io sell analytic tools to web sites. From what I can tell, it's mostly so that those sites can develop their own analytics for whatever it is that web site would like to collect. So the keen.io API itself is not evil, but I'm not sure that the customer couldn't use it to collect data in a way that might be.
It could be used for ad tracking, but it could also be used for something like just tracking how users use the site. There's an article at http://thenextweb.com/insider/2014/05/30/keeping-watch-pebble-uses-keen-ios-analytics-track-buy-use-smartwatch/ about how Pebble uses the tools to (anonymously) figure out how users are using their Pebbles watches. (Things like which features do users use the most. How long is the watch attached to the phone every day. What are users using the Pebble to buy. Etc.)
So, in general, allowing this site is probably fine, but sites that use this API may need blocked if they decide to do nefarious things with it.