Russia’s current operations in Syria are centered around the Latakia coast, and while they have made it clear they intend to help Syria fight against ISIS and other extremist groups, the group closest to Latakia is not ISIS, but rather al-Qaeda. That means, according to RUSI, Russia is liable to harm al-Qaeda, which is now a “bad thing.”
When the US launched its war in Syria late last year, they went after al-Qaeda with some of their airstrikes, which sparked condemnation for rebel factions who argued that al-Qaeda is part of the side the US is supposed to be helping. The US seems to be increasingly on board with that, with former Gen. David Petraeus openly endorsing al-Qaeda as the US ally of choice. Turkey has already been backing al-Qaeda against Syria for some time.
Indeed, it seems that much of the aversion to Russia’s plan to fight ISIS by getting Syria’s government and secular rebels together really centers on keeping the Islamist rebel factions, suddenly anointed as the good guys, on the outside looking in.