That is a simple adage and in the simplest circumstances it is true. In fact, though, it isn't necessarily true.

A customer might receive "free money," money for which they do not have to deliver anything in return. A vendor could choose to offer that with no strings. Though the vendor might have a hope of making money on that act, there need not be any conditions for the customer to receive the money. Free money.

But this would very likely be the result of a choice on the part of the vendor to promote their products by giving discounts (which would be the part that might be called a loss) in hopes and expectation of a future increase in business. Even if the vendor does not get new or more sales as a result of this act, it is not a loss if the vendor chose to do it. It IS indeed a cost, but because it's voluntary, it's not a loss. And it would not be accurate to call this a loss unless you're ready to say all business costs are losses.