Toxic being spread to all Pokemon is most likely because of animals and objects being able to carry diseases that make you sick. Bites from any wild animal can have rabies or bacteria which causes you to become sick.
The Komodo Dragon are well known for their bites since they have many different types of disease carrying microbes in their mouths that when they bite you, if you don't get some sort of shot right away, you could lose the limb just for that.
Then for the inanimate, you have bacteria that settles on dirty surfaces, when handled improperly you can make yourself sick.
I think that's why Toxic is a Universal TM.
There is only one other move that I can think of that can inflict Badly Poisoned and its strictly to Poisonous Pokemon.