Your question makes a presumption that the current USG has a calculating rationality about the best interests of America in its approach to the rest of the world.

This assumption diverges from observation quite markedly.

Tibet isn't the only instance where they are running the risk of annoying China for no obvious reason; their harrasment of Iran threatens a strategic resource partner for China who are beginning to invest heavily there.

I say obvious reason : These people are natural bullies and war-mongerers who like to create instability simply because they see profit in it. I'm not going all Naomi Klein when I say this but there have been numerous instances where their behaviour has had very negative outcomes for the USA, yet they continue. A repeated incompetence is indistiguishable from malice, so you may take your pick regarding motivation.

But Deep Throat was right : Follow the money.

keep to the Fen Causeway