I am not sure about Chinese culture, but it does not appear that Hu is bowing in respect. So it appears to be an overstated gesture by Obama.
In the case of the Saudi king, Obama's exaggerated bow was not only out of context for the culture, it was submission. That is the sort of bow that is never to be done.
He garbage in my book. What a disgrace.
And, in Saudi law, bowing to the king is illegal -- for exactly that reason.
Obama's bowing to the Japanese Emperor had the Japanese laughing out loud for at least a month. Footage of the scene became a staple on comedy shows.
Obama evidently pays no attention whatsoever to the Protocol Officers from the State Department and does what he wants to do. Which is to represent the U.S. groveling before its betters...