What prevents people (and Apollo) from simply using the same open APIs the the official reddit site/app? The requests would be all made from a client, so how w

What prevents people (and Apollo) from simply using the same open APIs the the official reddit site/app? The requests would be all made from a client, so how would they even know it's not their client? Is it illegal? Because I can make any reddit request with a curl, no? Please educate me.