In 14w06a, the mob AI was updated, but spiders and cave spiders lost the ability to see through walls. This is definitely a bug, because a quote from somebody on the Minecraft Forums proves it, and the quote was what Mojang originally said. Here it is: "The ability for spiders (and Cave Spiders) to see through walls was specifically added so they would not fall in the middle of climbing a wall.". And therefore, they are easier to combat.