Because a valid invoice (clearly stating buyers registered VAT number) is something needed by companies and a seller obligation in… all European countries. And VAT exempt because intra-community company purchases are exempt of VAT.
Can’t remember what I considered buying at Dozuki. Usually I just leave and give my money to another site that knows how company purchases work in Europe. VIES stands for VAT Information Exchange System and I suggest you take a look at: https://ec.europa.eu/taxation_customs/vies/