This is free because to install it, you must put a link on your page back to MixPanel. MixPanel likes links because Google likes links: it increases the domain authority and PageRank for MixPanel. You'll notice that alt text in the link back to www.pixpanel.com is "Real Time Web Analytics", surely a set of keywords that MixPanel is aiming to own in search.

This is a very clever and subtle SEO move that provides real benefit to users at the same time.