I am guessing it's because there are some who do not want to declare to the taxman any income generated via adsense. Especially in countries where there is already high tax, like the UK for instance. People in the USA complain about $3 a gallon for gas (petrol), yet people in the UK would love to pay that. Same goes for fags (cigarettes). So it is understandable that some do not want to declare, and because it is going to be paid in local currency, they (adsense people) think that they will be found out.
I could be wrong.