The risk margin is based on the assumption that the liabilities are transferred to another undertaking. To ensure, that the risk margin only covers the risks that are strictly related to the liabilities transferred the delegated regulation includes provisions on a hypothetical reference undertaking in Article 38. Article 38 Para 1 (i) of the Delegate Regulation explicitly asks operational risk to be captured in the risk margin.
This, however, necessitates to also reflect the premiums of the business in the operational risk calculation for the purpose of the risk margin. This because the premium volume is meant to reflect the volume of the liabilities which in turn is assumed to be related to the operational risk of an undertaking (the operational risk in the standard formula is less risk sensitive but formula based), as referred to in Article 107 Para 3 of the Solvency 2 Directive.
Indeed, even if premiums are not formally transferred to the reference undertaking according to article 38.1.d of the Delegated Regulation 2015/35, the premium volume is working as a crucial component in the assessment of the operational risk of the transferred portfolio:  premiums constitute risk indicators representing the importance of operational risk in the ceded liability portfolio.
Moreover, one has to note that the best estimate of liabilities can be small or even negative. Therefore, not taking into account premiums would, in such cases, not be sufficient to ensure that operational risk is well captured in the risk margin as required by Art. 107.3 of the Solvency 2 Directive.