I've always been curious if other arenas enforce this. Sounds like the Blues don't from the other poster.
I frankly hate this rule. Hockey is such an intense sport it's hard not to lean forward. Obviously you don't at SAP but it's always bugged me.
Whether leaning forward is an issue depends on where your seats are. If your seats are toward the end of the straight sections of seating (e.g. around the FO dots in the offensive/defensive zones - 116 as an example) and are low, people who lean forward can block your view of the game. If you are directly behind the net, at center ice, or high it is not nearly as big a deal.
How does a limit on height make the upper bowl steep. That's a width issue.
The footprint of the building is small since they have to keep it inside Santa Clara and Autumn Streets. Making it larger would take it closer to the street (I don't know what the building code is for how far it needs to be) and would likely eat into valuable and limited parking spaces. That combined with the height limit, as well as adding seats and boxes after construction had already begun, resulted in them cramming in as many seats as possible.