It's not so much that Moisins were sighed with the bayonet attached...
It's that they were issued with bayonets with no scabbards so there was no place else to put them.
And, the bayonet generally causes a change in point of impact, but won't generally change it by more than a few inches, if that, at 100 yards.
Oddly enough, I've seen a number of rifles over the years that have shot better with the bayonet attached. It's all how it changes the barrel harmonics...
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.