It's more fair than most payment systems. 

The fact that it's variable and changing based on demand is an indication that there is a real market happening.  It's much easier to have unfair bank fees, because as a client of your bank there is no immediate fee market on every transaction you make.   So it's more likely that any other payment system's fees are resulting in over payment to the bank/processor as they have less competition on their fees.  Or at least less dynamic competition.

Also the fee varies by the size of the transaction means smaller (in bytes) more efficient transactions pay less.  Large transactions does not cost the network more to process so should not be charged more.