It costs vendors to accept credit/debit cards, but most businesses roll in those expenses with the price of the product they are selling..
You will see "convivence fees" from vendors who cannot raise the price of the product for a variety of reasons. In the case of companies accepting credit card payments for taxes owed, it appears obvious that those companies cannot raise the amount of taxes paid to the Feds, State or local authorities, so they charge you a fee to make their money.

Just about nothing is fee free in the quest for MSing goodies. You pay for GCs, or MOs, fees on taxes, or loss of funds if doing buying groups or gambling sites.
As always, it is a decision that you make. Is the CC reward worth the cost in money and time??