Grand Nagus is the top political leader of the Ferengis. Quark was briefly the Grand Nagus. As such, he should be accorded high respect and good treatment when he operated the bar in DS9. Imagine if ex-President Obama opened a bar in China. I am sure the Chinese, from the ordinary citizens to their top leaders, will treat Obama with deep respect even if he is no longer President of the United States. Quark was treated disrespectfully by Sisko and Odo on several occasions. They do not talk to him politely.
How can this happen given that this seems out of line for DS9 as a matter of diplomatic protocol?