The Zonda exchange is a regulated exchange with an Estonian licence for "Providing a virtual currency service". Therefore, the exchange is subject to the regulations of the Estonian jurisdiction.
The actions of the Zonda exchange are taken due to the requirements under AML regulations, specifically the Estonian Anti-Money Laundering and Terrorist Financing Act of 26.10.2017. Nevertheless, national regulations in the European Union are rooted in EU regulations (AML Directives IV to VI).
In a nutshell, this is a requirement arising from the need to apply financial security measures in the form of monitoring the business relationship with customers with particular regard to the source of funds deposited with us (Article 23(2)(3) of the Estonian AML Act).