The code book is a wonderful thing. On one page it says one thing and on the next page it says something totally different. And just to make it interesting, the inspector will have a 3rd totally different interpretation of the code. Basically, to answer your question, somewhere in the code is an "exception" or a footnote or a comment or a drawing that will inform you that 1 1/2 drain is fine if youre talking about a tub/shower with single shower head.... but a shower stall would require the 2 inch drain.
The code book is not designed to be user friendly. There are codes that plumbers must abide by that are in other trades code books as well. Imagine plumbers having to follow rules that are in the HVAC or electricians code book. it happens everyday. Sometimes its amazing that anything ever gets done..lol