"algorithm" here isn't some fancy, hard to debug code. It is the business logic of weighting tweets and how recommendations are made.

There is great opportunity to abuse this by Twitter, yes. There is also a lot of money to be made. But in defense of some of that being secret, is the fact that any publicly known ruleset (with no hidden exceptions) _will_ be exploited by bad actors. Imagine if search engines told spam sites exactly why their site dropped in page rankings.