The general consensus of the travel industry seems to be "yes." For example, if I no-show for a hotel reservation, I get charged the first night's rate, including tax. It seems that the taxable event is the creation of a reservation. I'd be happy to be wrong, though.

I will say this: if UA is keeping the taxes, rather than remitting them to the government, then your theory is clearly correct and they should be refunded to the passenger, since UA is only acting as an agent in collecting them in the first place.