I am confused about Daemonology

The only 7th ed codices with psykers I have are Harlequins and Orks.

The rulebook says everybody gets Daemonology except Tyranids, unless their codex says otherwise, that using the powers cause perils on any double, and that Grey Knights and Daemons get only Santic and Malefic respectively but can use them normally.

Codex Orks says Weirdboyz get Daemonology. Isn't that redundant?

Codex Harlequins says Shadowseers get Santic Daemonology. It does not mention Malefic Daemonology. Setting aside both the flavor and the implication, does that mean that Shadowseers can take also Malefic powers because, as per the rulebook, their codex doesn't say they can't? (Mind you, even if the rules say it's ok, I think that if you're using Malefic powers with Eldar you should have things thrown at you. Uncomfortable things.)

Presumably perils still happen on doubles.

Based on these two codices (which is a very small sample size), it seems like GW established a system for who gets Daemonology in the core rulebook (you get it unless it says otherwise), but is using a different system in the army books, which seem to say if you can take it but not if you can't.

The BRB rulebook's listing of Daemonology was more or less a stopgap for the game at the time since prior to 7th, the trees didn't exist.  As the 7th edition codices (and FAQs) came out, they began to get more explicit with both trees, as is the case of the Ork Weirdboy, or a specific tree, as is the case of the Shadowseer.

Ultimately you have permission to use specific psychic powers in the rule book and the codex.  When there's a conflict, codex wins - so only the powers in the codex can be used.

As for perils - that stays the same as stated in the BRB with regards to which armies perils on which rolls when using the Daemonology tries.