Every society has social norms as to what is acceptable attire in given situations.  On our travels we enjoyed beaches where it was acceptable for women to be topless.  We travelled to another island where going topless on the beach resulted in a significant fine.  We encountered restaurants that refused service if we were not wearing certain attire.  The Manitoba Safe schools charter prohibits students from wearing "gang colours" at school.  
The wearing of the niqab violates many of the unwritten norms of attire and social interaction within our culture and therefore creates an instant discomfort and a negative emotional reaction among many people.  The discomfort with the niqab is a "gut reaction" that does not go away through intellectual argument.  My negative reaction to the wearing of the niqab in certain settings may change over time but in the meantime my "gut reaction" is vulnerable to exploitation.