Its not a bitcoin tax, it is an income tax.
Government does not know how many bitcoins one particular person holds or was holding, all they know is the amount of fiat that was transferred in to a bank account(yes banks do report on us), on that basis regular income tax is being calculated or at least it should be, by person directly responsible(you), in order to avoid financial punishment.
Its nothing new, rather obvious that when you take some profits, larger than income free limit, you have to provide your taxation institution with proper prof-papers(tax return) and adequate amount of ca$h