I'm not saying you're wrong, in fact I'm quite sure you're right in how things often happen.
However, for a service like Instagram, the ToS are a critical part of the business. Any changes made to the terms should be known, understood, and agreed on by whoever sets the overall strategy for the product. The legal team helps them encode it, marketing and PR help them communicate it outwards, but the strategy should be coming from the top, and the responsibility should rest there.
"Any changes made to the terms should be known, understood, and agreed on by whoever sets the overall strategy for the product. The legal team helps them encode it, marketing and PR help them communicate it outwards"
That's actually the sort of compartmentalization that creates these problems. "Whoever sets the overall strategy for the product" = the head of Product or the Product Manager for a specific feature. This person rarely has any legal or communications experience. If the legal team's job is just to "encode" what product wants, then that's actually where the problem starts.
The only real solution here is for Product, Legal and Communications to work together from the beginning. The product feature should be designed with Legal and Comm concerns in mind. Otherwise, the danger is that you'll end up exposed to exactly the sort of snafu that Instagram experienced here.
I'm not arguing for compartmentalisation, nor against the importance of all the teams you are talking about working together. There would absolutely need to be back and forth between legal and product as part of the encoding I am talking about, for example.
But it seems to me that if this was happening at even a basic level, then whoever is in charge of the product at Instagram must have known they were adding some pretty onerous terms. And if they didn't, it's a pretty massive failure to take the ToS seriously.
Neither option inspires confidence in a service that people provide with important data.
Really good points. Product is ultimately responsible for the user experience. The PM or head of product may not be a legal expert, but they better be an expert in knowing their users. Stakeholders signed off on the ToS or ToU. Each bears responsibility. However, Product is directly responsible for the outcome which is why a strong voice is needed - one that can say, "No Way!" when necessary. Of course, if the decision is made higher up the chain, then this just supports how deliberate the ToS change was - deliberate and worded exactly as it was intended. The wording was definitely unique enough that someone knew what was being asked for - legal expert or not - it was pretty clear.