For anyone too lazy to hunt it down, because even the source article isn't totally clear on it: The Open Skies Treaty, which consists primarily of NATO countries + Warsaw Pact countries, + the US and Russia, allows unarmed Arial surveillance of each signatory by any other signatory for the purposes of transparency regarding military movement and transparency. It was initially proposed by a Soviet Premier back in the 1950's, actually drafted by HW Bush, and began enforcement under the younger Bush.
en.wikipedia.org
I can definitely see how this helps Russia, as the weaker side of this agreement and the initiator of some recent land grabs, they wouldn't want other countries observing military buildups or anything else (unsanctioned military incursions into surrounding countries).
Of course, the treaty is only as good as the paper its written on. If they have not been holding up their end of the treaty, I imagine there are ways of applying pressure to have it enforced, rather than to simply withdraw from it altogether.