There are elements of their algo that I think should be openly defined, and perhaps there should be some regulatory branch that reports to Congress that has full access. However, obfuscation is often necessary to countering bad actors.
I think only if you offer twitter users the level of first amendment protection they'd expect with a government body. Otherwise reporting to congress would be an a bold faced circumvention the first amendment. Twitter is a privately held company with no need to report to congress.
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.