On ActivityPub, your homeserver is responsible for pushing your publications out and pulling your followed accounts publications in. All implementations need to work both ways.
In AT your homeserver (PDS) pushes your publications out to a service provider (BGS), and you need to subscribe to this BGS to get publications in. Content moderation is done on the BGS layer, so harmful content is not deleted, only hidden. This is because the founder is a free speech extremist. As BGS collect posts and create timelines, it's possible for them to make algorithmic timelines, insert ads and so on.
Because BGS are more resource intensive, fewer people will run them, when they exist, and the network will have a high degree of centralization.