First off, Fnatic-LDLC had to be replayed, but not because of pixelwalking. When they used the boost, some textures became invisible.
Secondly, Valve doesn't seem to care anymore about their own tournament. They released a CS:GO update while qualifiers for their own tournament where going on ... twice.
It's as simple as saying: "we know about this bug, but using it will have consequences". But they didn't, so as long as it is 'legal', it's not worse than a one-way smoke.