Here's what I got from airbnb cs in response to my inquiry about this issue.
I am sorry for causing confusion
I am going to send where it basically explains how this is working
As it stands there is no infants free program. As outline in he help center it is up to the host to either charge them as an additional person or not
If you do not want to charge them..click "family friendly" and mention it on in your listing description. What Airbnb does is to encourage guests to bring up this topic before a reservation is accepted
Maybe this is what happened in the case you have mentioned. A guest assumed that infants are free and he brought them to the listing.
By no means there is a button to click in order to take out the charges for kids and infants
Besides that, the policy stands that additional guests has to be announced unless it is arranged otherwise
So I assure you that you can go ahead and click "family friendly" and IB without being at risk
I hope I could explain it this time and I apologize again for the confusion caused