They are not walled for those going in.

How can we call everything a walled garden when many of them are free to get in and interconnect with each other?

The price of entry is very steep. Many people are willing to pay. Many are not. It's an exaggeration to say that many walled gardens are free to get into, I think. At least, I don't know of any.

You just have to give up your firstborn at the gates, and buy a paid Boost to be seen by people who clicked something like "notify of all posts."