As you seem to already know (but I find saying it explicitly help conversation), the purpose of taxation is government income. Taxation has a side effect of dissinsentivicing that which is being taxed. As a simplification, suppose income is only acquired through labor. In this case, taxing income is equivalent to taxing labor, and therefore dissinsentives labor. If we could transfer some of this task to something that we are more okay with dissinsentivizing (such as pollution) then we should do so. We could also view the dissinsentiving as a way of reducing usage of some government funded service and thereby reducing costs. For example, the gas tax is effectivly a tax on road usage. In this perspective, taxing is similar to the government charging for a particular service. Of course, given either of these perspectives, and internet tax is still stupid.

The reason that income tax is generally preferred is that it allows much more government income without effective incentives significantly. This is because the marginal utility of money seems to decrease, so for a linear increase in 'utility-payment' laborers receive a super-linear amount of money; and we can therefor tax it progressively while maintaining a constant incentive burden. This is a clear case a price-discrimination which is far from "natural" in a pure economics sense of the word.