THX offers certification programs for all kinds of things - including cables, speakers, computer speakers, amps/receivers and even media. The requirements are different for each.
I used to have a Logitech 4.1 computer speaker set (z-560 or something). It was THX certified but the requirements for that certification are not even remotely related to the requirements for HT speakers. I would agree that THX certification for some things is just silly but believe certification for receivers and speakers is useful, if not absolutely required, to have a well built, high performing piece of equipment.