First of all, there is a fundamental difference between a "tax" added to a can of coke and a "fee" added to an airline ticket.
Secondly, the problem is that airlines are labeling elements of the fare as "taxes" and "fees" which are, per legal definition in some jurisdictions in which the terminology is used, neither "taxes" nor "fees". As a result, what they are doing is unlawful price advertising and/or price labeling.