Well if they did say 'stop throwing stuff or there will be a forfeit' people would've stopped
Also this paragraph from the Grantland review pretty much 100% speaks to why I didn't even care that people threw stuff
Sure, throwing stuff onto the field isn’t going to accomplish anything, and when you consider how much it probably hurts to get hit on the head with a beer can thrown from the upper deck, it’s also more than a little unsafe. But despite being warned against it, and being aware of the futility and the danger of it — to say nothing of the fact that we’re conditioned from childhood not to literally throw things when we’re upset — hundreds or even thousands of fans threw things onto the field.
We’re supposed to have evolved beyond displays of incoherent rage. We’ve suppressed that instinct in ourselves and covered our world in rules to try to prevent that rage from bubbling to the surface, but from time to time, we’re confronted with an outside stimulus so bizarre, so unexpected, and so enraging that it seems like societal order shouldn’t allow it. But bizarre (and, frankly, unfair) as it seemed at the time, it happened anyway, and there was nothing anyone could do about it. Civilization failed us, and when civilization fails us, a civilized response just isn’t enough. It wasn’t right, or smart, to throw things, but it was the most human thing to do.