There's actually a bit more of the sewer backgrounds on the disk, but they aren't playable. Quality-wise, they're green and lack about as much detail as the mines in RE1.
As for stripping the zombies the ability to climb, they probably removed it since it's VERY easy to take advantage of. If you downloaded the hacks that were provided on THIA, you can fight some in the sewer corridors and you'll know what I'm talking about. Areas in retail were designed to be claustrophobic and without the climb function in mind, so the zombies were just as menacing even without it.