If you spy on the UN, you own it. You have a disproportionate advantage in all negotiations (unless other countries are also effective at spying[1]). You might think that it is fine that the US effectively owns the UN, but its credibility is ruined.

The compromised UN is effectively a body for representing and legitimizing the wishes of the cabal of US allies as if they were the actual consensus of all UN member states.

[1] But it is still not possible for any nation to be more effective at spying than the US, given the UN's location.