The function of the 5.1 or 7.1 analog outputs on a Blu-ray player is very similar to that of a preamplifier/processor. A preamp used with a DVD player for example would take a digital input such as Toslink or Coaxial and convert it to analog for output to a power amplifier. A DVD you may be watching may have DTS for example, the the pre/pro receives the digital bitstream of DTS audio from the digital source (dvd player) and converts it to analog. The amplifier isn't getting DTS, it's getting 5-7 (LFE goes to subwoofer obviously) channels of analog audio (because the amplifier is analog, not digital).
Use the same principle logic with the Blu-ray player, only instead of the Blu-ray player sending a digital signal out it's decoded within the player. The player is now taking the action of a digital processor. The preamp receives analog from the Blu-ray player. The preamp's job at this point is level control, and that's pretty much all it's doing. It's not doing any additional decoding.