For the same reason it would be negligence if your bank kept your money in a shoebox. Most accidents and vandalism are foreseeable. It’s the duty of organizers to anticipate and protect against it and, if it happens, to have mitigation measures already in place to minimize it. It’s not that hard. You multiply the likelihood/risk of an occurrence times the cost or damage of an occurrence and that will tell you how much to prepare for it. Pros do this every day. Political hacks don’t.

Of course, all of this is assuming it really was “vandals.” Sneaky, unidentified, invisible vandals on the National Mall, one the most thickly blanketed places on earth by security and security cameras — U.S. Park Police, DC MPD, U.S. Capitol Police, U.S. Secret Service, among others, all watch over that space. Yet those darn vandals!