Are spectrum auctions a tax?
Suppose you write some software that you can then freely sell at practically no cost per unit (the wonders of the internet). You want to make as much profit as possible. Since you have almost no per unit cost, maximizing your profit will amount to maximizing the revenue, the price per unit times the number of units sold.
Note what did not enter the calculation: The cost of developing the software. How much money you might eventually get might influence whether you put in some effort to create the software, but it will not influence your pricing decision once you made the software.
The companies bidding for spectrum are essentially in the same situation. Once they have gotten their spectrum rights, how much they paid will not influence how much they charge..
Of course someone has to pay, but these are companies bidding for monopoly rights. Apart from the cost, the license gives them a huge profit and this profit is what the cost is paid from.