There are tons of things that are paid for on a percentage basis, and the "is this value added or not" line is pretty difficult to define clearly.
It doesn't make sense to think of it as a tax, really, sure taxes can be used to provide services (or other things) but the fee being paid here is directly tied to the service you are receiving. If you want to draw parallels with government, it would better match a toll road or bridge; you can still argue the total collected toll doesn't fairly match the cost of providing the service, but the mechanism is at least clear.
There are tons of things that are paid for on a percentage basis, and the "is this value added or not" line is pretty difficult to define clearly.
It doesn't make sense to think of it as a tax, really, sure taxes can be used to provide services (or other things) but the fee being paid here is directly tied to the service you are receiving. If you want to draw parallels with government, it would better match a toll road or bridge; you can still argue the total collected toll doesn't fairly match the cost of providing the service, but the mechanism is at least clear.