Nothing is free, not even Pix. It had costs for development and has costs for running it. If it's free for the end user, then somebody else is paying.

As a physical person, you can receive 30 Pix payments per month without having to pay a fee, but send as many as you please.

A business however can be obliged to pay fees for receiving Pix payments from physical persons (ie customers), and when using the more advanced Pix features.

Banks very likely choose to not put any fees on Pix payments for their business clients for now, since it's beneficial to keep their money flowing in. But that's not different than European SEPA payments or American ACH payments being "free" for businesses.