Is this a regular ploy by airlines to offer "free" flights but then to claw it back through inflated taxes?
Airlines have no control over taxes, but you are being very naive if you think airline mileage programmes are there simply for your benefit.
They are a loyalty programme, and just like similar programmes you see in supermarkets and elsewhere. Their sole purpose is to provide the business with another source of data they can analyse.
The bonuses and perks provided to you through the programme are nothing more than a carrot being dangled in front of your nose to keep you incentivised to carry on pushing data into the programme for them.