in german law a tax is general purpose, government can use the money to pay for any of their "services". whereas a fee has a specific purpose and cannot be used for anything else.
Of course, sometimes fees and fines can be seen as "stealth taxes" when they intrude too far into everyday life, particularly when state monopolies are involved. For example, the cost of First Class postage with USPS is technically a fee for an optional service, but the USPS monopoly on First Class letter delivery makes it at least partly a tax, to the extent that another carrier might have provided the same service for a lower price in the absence of the monopoly.
In Germany that may be true, but I was speaking of the US, where the terms are used differently. There is no expectation here that "fees" can only be used to defray the cost of providing specific services.
I agree.
But I'm guessing that it's the same in Germany. They call them "fees" but they're really taxes. Except that they can't be used for other purposes.