When you summon a wither in survival, the wither doesn't attack the player or anything nearby example: pigs, horses, etc. This is a bug when a wither is summoned via command line.
Steps to reproduce
Be in Survival.
Summon a wither. (Easy-Hard mode. Cheats Enabled.)
Notice that the wither is just hovering.
The Wither does shoot out a few random skulls, but it just doesn't really seem to have any focus. Actually, a Wither that is summoned via /summon doesn't fade from being white when it is first spawned, it doesn't have a weak-shield mode, and it doesn't shoot blue skulls. It's as if a /summon-ed Wither is missing some of its behaviors or something.