A totally plausible explanation why the Kremlin-controlled 'Vostok Battalion' has been active in Eastern Ukraine is provided by the excellent Mark Galeotti in his blog.
Some thought-provoking comments too..
Radio Free Europe/Radio Liberty today also provide illuminating information in their article "Vostok Battalion, A Powerful New Player In Eastern Ukraine"
Moscow’s strategy of chaos has worked too well, eating away not just at the cohesion of the rump Ukrainian state but also the emergent East Ukrainians, too. There appears to be increasing evidence of disputes between militias, and between the relatively professional defectors from the Ukrainian security forces, the opportunist thugs, and the “war tourists” from Russia. [And local mafia gangs ..LEvko] Thus, Moscow’s hopes to be able to cut a deal with Kyiv–regardless of whether Poroshenko can and will offer the Kremlin what it wants–depends now on bringing order to chaos. The Russians wished for chaos; now they know why so many folk tales warn of being careful what one wishes for…