Nathan, before DCP became widely used for movie projection, many cinemas were already using video servers to screen preshow content. I think that now most commercials and trailers are on DCP and screened from the same server/projector, but maybe there are legacy systems around.
But I agree with you: in today's world of automated cinema booths, there is absolutely no reason for the trailers/commercials to be the culprit, as the playback level can be set differently for pre-show and show content. It might all boils down to lazyness and/or disapearing projection operators, fear of customer rants (too loud!), and ignorance of the "too loud" cycle by stubborn/stupid/egomaniac directors and producers.
As for the previous comment about "damn engineers wearing earplugs" (not by you Nathan), first thing I'd like to say is that they are "mixers", not engineers...
And when faced with dictatorial directors (hey, each movie is a sort if small banana republic complete with it's own dictator), it's either lose your hearing or lose your job... or wear earplugs. The cinema-goer will only be put through the high SPL once (for two hours straight if it's a M.Bay movie), but the mixers have to bear the insane levels asked by the fool director for 9 hours per day, for multiple weeks. Wouldn't YOU wear earplugs??
I'm not defending them, because they should try to educate their directors more. But on the other hand I do realise that even suggesting to a high-power director or producer that his ideal 747-at-liftoff-intimate-scene can very easily lead to dismissal, even for very experienced sound guys.
What's amazing is that even in Live sound, I believe that there are loudness rules emerging to protect the audience. God knows we've all been to concerts that were so f*****ng loud that we would wonder WTF the FOH engineers were doing. What we have right now is that the cinema owners are taking things into their own hands, but with no education or knowledge of why/what/when in sound. It must be said that not many concert producers have had this sort of attitude.
I'm slowly accepting the idea of loudness measurements for cinema, as long as we can still mix with as much dynamics as we like. Some things need to be a tad loud, but never insanely loud. So when the ball starts rolling with the SMPTE or AES or whoever is needed to write the rules, I think we should all get involved as much as possible.