The item called free may be purchased with information the buyer does not know they are selling.

Free usually means you are paying in something other than money. Attention, data, habits, the ability to be reached later, all collected in exchange for something that presents itself as a gift. The price is real, it is simply not denominated in a way that shows up as a cost.

When something is offered free, ask what the provider gets from it. There is always an answer and it is often stated somewhere. Knowing the price does not mean declining, only that you are trading rather than receiving.