Eurocontrol already has fees for ATC usage. This fuel tax is presumably in addition to that.
> They can’t impose a tax on jets that use their airspace?
Presumably they already signed aviation treaties that do not allow for this.
Yes, there is a tangle of 5000+ Air Services Agreements which, as international treaties, supersede national laws. Almost all agreements include mutual tax exemptions on anything not unloaded at a destination, including fuel.