some criticism was politically motivated and some was self...

some criticism was politically motivated and some was self interest but remember how the tax was originally framed and how poorly sold the scheme was (Swan infamously didn't understand WACC and denigrated the entire mining industry for not paying its fair share)

miners were to pay excess tax on any return over the bond rate: that was inequitable as it didn't reflect the risks involved with mining and people didn't see it as being fair play

if they had struck it at a higher return it would have defused the whole situation and probably been a vote winner

it was also proposed at a time when most of the governments big projects were turning to custard so people were loathe to give them a bigger tax take to fritter away...if the revenues had been put into a sovereign wealth fund it may have succeeded