The Treaty on Open Skies was signed in Helsinki in 1992 and took effect in 2002. Signatories to the treaty must allow other members to overfly their countries in the hopes that, if everybody knows what everybody else is doing, war will be averted. The overflights must be carried out with unarmed aircraft using video and still cameras, infrared scanning devices and certain kinds of radar. The countries that are being flown over have the right to inspect the surveillance aircraft to make sure that the provisions of the treaty are being followed.
Both the US and Russia have carried out surveillance flights of each other since the treaty took effect, but with the current rising belligerence of Russia, the US is concerned about an uptick in the number of Russian flights, and what they might be looking for. It’s very likely that Putin is doing it simply because he can, but there’s really very little the US can do about it. Putin never misses a chance to push the world’s buttons.
Tupolev Tu 214R photo by Rimma Sadykova via Wikimedia Commons; Boeing OC-135B photo via US Air Force