What's The Point of Flashing a Deck If Some Idiot is Gonna Compromise It?

So here's a deck I looked at in Shell Beach CA; it was doomed to die the day someone cut the flashing to install the overflow. The flashing was cut right down to the deck level, leaving water no choice but to run into the deck rather than away from it and over to the drain. RIP ole buddy. 

This is the kind of hack work that will keep us busy for years!

Standard Practice in Sheet Metal Work: A Reference Book 

Architectural Sheet Metal - 7d Thermal and Moisture Protection ((Brochure - 20 pages))

Residential Sheet Metal Guidelines