How are swarm lords different from Eldar Farseers with spirit stones (can 2 psychic powers in a turn) or even Eldrad with his staff (3)? If a specific special rule or widget over rides the default base rule, that is hardly unusual. That's the way the game works all too often.
I've had a similar debate about Psychic powers and certain IC's being able to use more than one in a turn. (In my case, it was Njal Stormcaller).
If you read the Psychic Powers section in the rulebook, it says that if a model (that is capable of using Psychic Shooting Powers) is able to fire more than 1 weapon during the Shooting phase, then it can replace each of it's normal shooting attacks with a Psychic Shooting attack.
So in the case of monstrous creatures who have multiple shooting attacks, if it is capable of using Psychic Powers, it can use more than one during the shooting phase, so long as he:
A) Has at least 2 different shooting attacks
- and -
B) Has at least 2 different psychic shooting attacks
Note also that no model can use the same psychic power more than once during any turn (unless otherwise specified in a codex).
Farseers with Spirit Stones and Librarians with an Epistolary can use 2 Psychic powers in a single turn, but never more than one in a single phase (Phases being "Movement, Shooting, & Assault"), because (and correct me if I'm wrong) Farseers and Librarians only have 1
normal non-psychic shooting attack. Therefore, since you can only replace one non-psychic shooting attack with one psychic attack, that is why Farseers and Librarians can only use one per shooting phase.
