If you were trying to buy groceries and had no interest in Instagram but you were forced to create an Instagram account to buy the groceries, that would be a problem, but this type of comments complains that they like the content but find it unacceptable to be asked to open a free account to consume it. God forbid someone asks money for a service people want to use: how dare you ask for payment for a product I want to use.
Yes, they built the structure but it's an empty house without the users content.
If they don't know who you are, they can't send you hyper targeted ads and make more money from you. They probably get a lot of people to sign in by blocking content so it's worth it to lose the smaller group that gives up and leaves.