There is no hidden transaction, you signed up for a service to:
Upload your content so they send copies to whoever asks.
In exchange, they used to show you ads, and that was fair. Then, they started collecting your browsing data and selling that too... that is a second transaction, and got regulated. Now, they are selling a service of bundling together the content people asked them to share in the first place.
In your analogy, you asked them to send your nuts and bolts for free. In exchange, they advertised stuff to you. Then they started collecting the addresses of your clients... that was not fine. Now, they're throwing nuts and bolts from multiple people into a box and selling it as a "sampler kit", nuts and bolts you did ask them to send for free.
Did you not understand the value of your product? Maybe, but you asked them to do it anyway... and you're doing the same by posting content right here. 🤷