You're not the President
"Let him", indeed. He can put up just about anyone he wants, but Bolton is hardly someone Bush pulled from the crowd at his last election rally. Democrat or Republican, the President appoints those who he thinks will serve America's interests best, just like other countries appoint those who will serve theirs. The US is therefore acting no differently to all other UN members in that regard. You can certainly argue Bolton might not be the best choice, but don't try to make it sound like the US alone is breaking some international gentleman's agreement or the President's obliged to appoint or not appoint according to your preferences.