It really is a tax on global revenues. It's a tax on global revenues that is enabled by the fact that EU residents get services for free, which enable the global revenues. But the companies are also paying a VAT/GST/sales tax on some of those revenues in nations that charge them.

Let's use lumber as an analogy. Let's say the trees are in nation A. And a company from nation B is cutting them them down to make lumber (and is paying appropriate taxes in A for licenses, employees, land, etc). Lumber which is sold to companies in nations C, D and E. And appropriate taxes are paid on those sales. The company also pays income tax in nation B. The EU tax is like nation A saying, "you're using our trees to make even more money in nations C, D and E -- pay us a share of those sales!" In this analogy, the users are the trees.

To use another analogy, it's like demanding farmers pay taxes on the revenues that McDonalds makes. It's just silly.