It's free because Facebook wants the training data. Pretty obvious I would have thought.
If you're ok with helping Facebook improve its AI then I see no reason to worry about the fact that it is free.
Our vision is to make it easy for developers to make apps or devices that can leverage AI/NLP/Speech. As you probably know, the key is the amount of data you can collect. The more data, the better your models will be, the better you API is. When you are a startup, you need at some point to generate revenue to continue to grow/hire even if you objective is to gather more data
It's just like Twitter saying "We don't need to generate revenue directly from our API and that's why we're opening it all up because what's good for the community is what's good for us!", except that this isn't 2006 and everyone knows better.