It's all about creating the appearance of wasted effort on the side of the taggers, possibly sprinkled with a little political nudging to stop highlighting that specific bit of failure.

I hate tagging done in places where people obviously care about the building the walls are part of, but in this case it's hard not to see this as a public service:

“Look at this hubris. Hard to ignore now isn't it? A huge newly built tower abandoned for all intents and purposes. It's a waste of space, money, and it raises questions about sustainability and fairness.”