Why?
His official title is the President of United State of America.
Not President of Trump Corporation.
That is the difference.
If you are disrespectful to the President of USA you are basically saying you hate the entire population of USA … including all the multiple ethnicities.
Remember in USA you have Democrats who you might have same ideology with but when you disrespect the President, you also turn your back to the people with the same ideology as you because they are also being represented by the same President.
Do you think the entire USA, Russia, North Korea, Iran have only one ideology?
Show respect and the world would show respect in return.
Are you seriously suggesting that any head of state should be respected? We should have respected Saddam, Stalin and so on?
If he is on official visit (if you invite them) then you show respect to them, if you do not want to respect them why the invitation?
Show respect in a civilise way and you might change them.