Most surgeons require their patients to lose weight to some extent, some up to 10% of the execess weight pre-op. There are those who put their patients on liquid diets and/or pureed diets pre-op, too. All this is, as you said, to reduce the size of the liver, but some also seem to consider it a kind of test--the final one--to see if you are really commited to this surgery, and willing to do everything that is required of you post op to insure the success of the surgery.
It's OK to vent

but each dr's protocol is different, and the difference all comes from the personal experiences of the individual surgeons. So, it's important to follow your surgeon's protocol (even if it's different from your next door post-op who has a different surgeon) becasue he/she is the one who knows your body and system, and understands fully how they changed it for you.
Good luck!!(((((HUGS))))))