Because all other companies are being taxed when offering products or services to citizens of the given country, within the legislation of that country. Certain Internet companies use questionable tax planning strategies (in Europe as well as the US) to pay a ridiculously small amount of taxes and thus gaining an unfair advantage.

> Or Why should taxes exist at all?

Because running a country costs money which is required to provide services such as law enforcement and a regulated banking system. These services are required by citizens and corporations alike.

But going further down this argument is obviously not in the scope of this discussion thread and will not end up being a fruitful conversation. We're talking about a small-ish change to the tax system rather than a full blown anarchist revolution here.