Any agent/soldier can access it without a complicated permission or authorization structure. You don't need access to some spy satellite. All you need is a mobile phone, tablet or laptop. It would seem very effective to me.

These marks could be set by ukrainian citizens wanting to return the glorious days of Soviet Union.

Its easy for local collaborators/spies to use this kind of system for communication with the Russian army.

No special skills or tools needed, and the on-line behavior is really hard to distinguish from 'expected' behavior. Its a pretty nice, and really easy to use, covert channel.

$cheap

There is a saying in Russia, at war all measures and ways are good (на войне все средства хороши). Whatever can be used will be used either due to convenience, due to malice, or due to any other reason.