It feeds the victim-hood narrative that some have, and that is regrettable. But they found reasons to feel unjustifiably aggrieved before spray-paint was invented. But, for me, it just isn't even remotely on the level of the monument itself. If I grew up in a place that systematically encouraged violence against me, and built monuments that justify it, I might be pretty upset to, to the point of doing something illegal like scrawling a message, out of frustration and a need to put out a truth that no one wants to hear. I can't find it in myself to fault that reaction.
Call me chaotic good.