Fact is, as daft a concept as it might be, to shoot it down when it only slightly strayed, its not. If it has been happening repeatedly you cant let it carry on. The thing is, particularly when its Russia, you cant let them take an inch.
There is no way to know that the next time a plane strays its actually strayed, it could be on a bombing mission to your capital, you have to draw a line.
Russia have probably been taking the piss and did it one too many times.