The blockchain which propels Bitcoin is anonymous in nature and it is not centralized which make it very difficult to collect tax from any transaction that is carried out in blockchain.
If you were using a TenX or Centra card i would understand that you are paying tax for a physical product at the time of purchase. But its interesting to what degree law is going to intervene and try to regulate with personal bitcoin income, especially if shops started accepting it directly? Its almost better to keep in the currency digital than convert back to fiat?