This is done in order to prevent extra questions and document checks at the consulate, as well as audit of the inviting company and possibly the visa agent by the authorities, since the inviting company is based in Moscow, and not Sochi.
We understand that some of your clients actually visit these regions on tourist or business purposes. We would like to reinstate that the cities to be visited are not mentioned on the visa itself, and mentioning or not mentioning a city in the invitation does not impose any restrictions on the client's route. It is perfectly fine to mention Moscow in the invitation, and visit Sochi once in Russia.
But please be warned that an invitation from a local company or hotel booking confirmation can be required at the Customs in Sochi or another city of Krasnodar region upon arrival.