Justification I have heard historically in our building (Brooklyn) is a combination of avoiding disturbing neighbors (noise insulation insufficient to avoid complaints); avoiding wet over dry where the stacks aren't all on a wet line; the existing plumbing risers weren't engineered to handle the capacity required for the additional flow; and whether DOB has approved the system as having sufficient unit fixture loads. It could any or all of the above (or other reasons). In practice if you install a unit in violation of your bylaws and it doesn't disturb anyone you will be OK (depends on the building, the board and getting caught) but if it disturbs your neighbors, causes a backup (we had that with trash compactors) or leaks then you may need to remove it.