Because he knows more about etiquette than you do.
I didn't know it's etiquette for a US President to bow down to other presidents/premiers or dictators. If it's an Asian custom to bow down, then since the Chinese guy is visiting the US wouldn't he bow down to Barry?
On the other hand, isn't that picture of Obama bowing down an old one from last April? Although it doesn't matter because the summit was held in DC in Obama's hood. But I do wonder if Barry did it again this time.