I’m not sure that Obama’s decision to “snub” Putin at the G-20 meeting will be all that important in the end, but Jacob Heilbrunn is right when he says  this:
The more Obama seeks to challenge Putin, the stiffer Russian resistance will become.
This is a point that is often lost on Americans and Westerners that demand that their leaders “stand up” to Russia, since many people in the West seem to think that the Russian government would behave in a significantly different fashion if only more Westerners did more to shame or “punish” it. We wouldn’t expect our political leaders to respond to a foreign government’s pressure by making concessions and heeding its demands, so it’s strange that so many people seem to think that Russian leaders are going to be impressed by increased pressure. If the goal is to get Russia to make changes or concessions on any issue, we have seen over the last decade that this isn’t the way to do it. There are some issues, especially domestic Russian ones, where foreign pressure is not just ineffective, but can be genuinely harmful to the causes it is trying to aid.
As rebukes go, Obama’s cancellation of the bilateral summit meeting with Putin isn’t that strong, but it could and probably will be used as a pretext for greater antagonism on some issue. It goes without saying that the decision to cancel will have no positive effect on Moscow’s internal conduct, its asylum decision on Snowden, or any other outstanding disagreement between Washington and Moscow, but then it isn’t intended to have that effect. This is a decision primarily meant to placate American critics of Russia, who predictably won’t be satisfied with this gesture, and to save Obama the trouble of a meeting that would likely have been fruitless anyway.