The APIs definitly used to be open enough that you could hit a "Generate token", hit one endpoint with cURL and then receive a firehose of all public tweets from that moment on, no reviews or validation at all, all you needed was an account + token.

I think this is a huge reason for the initial popularity, because it was trivial to build really fun experiences on top of that, until they cut it off for whatever reason (guessing money, one way or another).

At the same time, you could also view tweets without being logged in, and you saw replies too.