Ummm, no again, Bambino. I do not think dressing accordingly in bars is an Islamic rule.
High end bars and clubs in major cities all over the world, in general, impose DRESS CODES. I have friends who own clubs in Manila and Tokyo, these clubs have dress codes. Both to men and women. Yes, the reason in imposing such rules may be different as to Doha's, but the premise is all the same. You want to know why?