If the only payment method is bitcoins, I don't understand why they tax the purchase with VAT.
VAT only applies to FIAT transactions. With Bitcoins you can avoid this tax.
This is not true, at least in my country. VAT applies to all transactions, for goods and services, no matter what the currency it was originally taken. For the purpose of VAT the value of the transaction is recalculated in the local currency, and VAT (20%) is always applied in local currency. Avoiding to pay VAT is serious offense and actively prosecuted, so sport of avoiding VAT is not even talked about in any business worth its salt.
You are wrong. Bitcoin is not a legally accepted currency.
When you operate with Bitcoins you are "exchanging" them for good and services.
Strictly speaking, you only need to pay the VAT when you convert from fiat to bitcoins, because you are buying them. Whatever transactions you do afterwards is bartering.
Do you understand now why governments hate bitcoins? It is not because of money laundering: it's because it could be legal to use them to avoid taxes tributes.