I can assure you that the government only said that because it's thinking of making money out of Nigerians trading bitcoin - no more, no less.
They can only make money from the exchanges through tax which could increase transaction fee. the only way to sideline the government is to have exchange into your currency outside the country to avoid government intimidation.
Regardless if you exchange your bitcoin inside or outside the country you will still have to pay taxes if you incurred income. That is income tax. But if indeed bitcoin is taxed per transaction in the exchange then that would be the tax we could avoid if you use an exchange outside of your country. So I think it's really impractically for governments to tax bitcoin transactions in exchanges as people would just shift to another exchange outside of that country.