Why do death cap mushrooms not warn of their poison?
Most of this type of evolutionary question are generally worthless, IMHO. You can rationalize anything post hoc and then go away with the misapprehension that you have been involved in science. 

To add a few of those possible post hoc explanations (which took longer to type  out than think up):

1) The poison is only accidentally a poison.  It serves some other function in the mushroom's metabolism.  There are numerous possible examples of this.  For instance, the stalks of rhubarb are edible, while the leaves are poison; potatos are edible unless they've turned green from exposure to sunlight...

2) The poison might not be directed at humans.  The mushroom might have some warning that would be obvious to other species, for instance a smell or markings visible in UV (such as many flowers have).  We have lots of examples of poisonous plants that we grow because they're attractive to us: datura, lily of the valley, foxglove, oleander, delphinium...

3) If I'm not mistaken, mushroom poisoning takes rather a long time to kill, which argues against it being a defense.  The poison might have evolved as an aid to reproduction.  Mushrooms are the fruiting body of the fungus.  If an animal eats it, it consumes the spores.  If it dies hours or days later, it has transported those spores to a distant location, and the corpse provides nutrition for the new fungi.

However…

Doesn’t the all-white toadstool look distinctive and memorable to you? It doesn’t look remotely like the field mushrooms or other edible mushrooms I would collect. What edible fungus would you confuse it with?