Why is this free?

Is this a loss-leader to get people to sign up for the full Mixpanel service?

These days I just don't trust free services. I don't think there's anything nefarious about them, I just don't trust that they're going to be around in a year (why keep running a service that has costs but no revenue), or that they won't get bought for their talent and promptly shut down.

If it uses resources to any significant degree (a JS on every page load can definitely do that when used at scale) there should either be a fee or a damn good reason why I should put my faith in it, especially if you're asking me to base business decisions / process on it.