Every service / product, so also transportation (bus / train / car fuel) is taxed, called GST, Sales Tax or whatever.

Why are air tickets exempt from sales tax ?

According to COP21 climate change plans, we should do everything to lower CO2 emissions and air travel is one of the larger ones. Now, oil prices are low, air tickets are cheaper than ever, this is a chance to tax air tickets, according to the principle 'the polluter pays'.