I was doing some testing in the latest snapshot, and when summoning in a Stray, he had no Bow. Occurred with the other two types, but when leaving off the "SkeletonType" tag, the Skeleton that was summoned came with a Bow. Tried this with Zombie Pigmen, and indeed, if summoned with the "Type" tag, they do not wield a sword, and if it's left off, they do. Used Strays as an example. Left: Spawn Egg, Right: /summon

EDIT: Tried this in 1.9.2, and it still happens, so I don't know it this is truly a bug, or where the game disregards natural tags (Skeleton w/ Bow) when an artificial tag ("SkeletonType") is used (Skeleton w/o Bow).