Why should we consider it a tragedy that people were occupying this building? I certainly don't see any reason to, just like I didn't see any reason to consider the occupation of the two police stations (and destruction of one) a tragedy.

The symbolism is what's important. This building, the largest and tallest and most ornate of all the government buildings in DC, is the beating heart of the US government. To take it over is to strike at the very heart of the government, and to have one branch taken over by instigation by the other branch, it is horrific. Because the consequences are a collapse of this society. With no functioning US Government, the United States cease to be united. It's an existential symbol, and that's why terrorists attempted to target it on 9/11 and why terrorists targeted it this week.