This is so wrong, it couldn't be farther from the truth. Lye is lye is lye. There is no difference chemically between the lye used for food processing and the lye used for cleaning drains: the difference is SOLELY in the concentration of the lye solution.
Lye used in plumbing is simply a strong concentration of either Sodium Hydroxide or Potassium Hydroxide, either in crystal form or a thick gel. It works because the chemical reaction between the metallic alkali and fat or oil (saponification) converts the esters in oil or fat into soap. Lye used in food processing is simply a weak concentration of either Sodium Hydroxide or Potassium Hydroxide.
Lye is hazardous, but it is NOT poisonous. Lye is a caustic base and, in high concentration, is highly corrosive and must be handled with care.
There are two forms, Soda lye (sodium hydroxide) and potash lye (potassium hydroxide) You are correct, chemically they are all the same, but I am sure for food preparation there is a strict guidline for purity in what can and cannot be used. There are many different purities of Lye.
In terms of classification as a Hazardous chemical or a poison, it depends on the definition you are going by. According to www.M-W.com
a poison is defined as:
Main Entry: 1poi·son
Etymology: Middle English, from Anglo-French poisun drink, potion, poison, from Latin potion-, potio drink -- more at POTION
1 a : a substance that through its chemical action usually kills, injures, or impairs an organism b (1) : something destructive or harmful (2) : an object of aversion or abhorrence
2 : a substance that inhibits the activity of another substance or the course of a reaction or process <a catalyst poison>
Lye solutions can be used to dissolve tissues and kill organisms. I would say that chemical action warrants it's definition as a poision, however I don't think this involves the breakage of covalent bonds. The term "poison" is usually used to refer to direct reactivity that involves covalent bond reactivity or things in nature, such as snake venom.
In terms of using Lye in food preparations, I found this at the oracle: (www.wikipedia.org
"Food uses of lye include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel color production, poultry scalding, soft drink processing, and thickening ice cream. Olives are often soaked in lye to soften them, while pretzels and German lye rolls are glazed with a lye solution before baking to make them crisp.
Specific foods processed with lye include:
The Scandinavian delicacy known as lutefisk (from lutfisk, "lye fish").
Hominy is dried maize (corn) kernels reconstituted by soaking in lye-water. These expand considerably in size and may be further processed by cooking in hot oil and salting to form corn nuts. Nixtamal is similar, but uses calcium hydroxide instead of sodium hydroxide.
Hominy is also known in some areas of the Southeastern United States, as the breakfast food grits, dried and ground into a coarse powder. They are prepared by boiling in water,with the addition of butter and other ingredient to suit the tastes of the preparer.
Sodium hydroxide is also the chemical that causes gelling of egg whites in the production of Century eggs.
German pretzels are poached in a boiling sodium hydroxide solution before baking, which contributes to their unique crust."
As Harv pointed out, it's all about the dose. I would also like to add that purity is also a factor.
And I think good pizza can also be hazardous. It's called CORONARY HEART ATTACK!