Actually there is evidence that plants do register pain. They have anervous system, just not an animal-like nervous system.
When you rip off a leaf or pull up a carrot, chemicals and hormones are released that spread throughout the organism. I've also read research (that i can't sem to find online, so assume it was in a journal) that there is also an electrical response induced in injured plants.
I've even seen some research looking at electrical communications between plant roots via ground water. Plant neurobiology is still a young science though.
The simple fact is that because people cannot observe a reaction to pain that they recognise in plants, as they would in other animals, they assume that plants are unaware, they are not. The evidence is growing that plants are just as aware of their surroundings and of their own bodies as animals.
Many people justify killing animals to themselves because the animals are "inferior to us, they aren't human". The vast majority of humans use the same logic with plants "plants don't feel pain, they aren't animal, they are inferior", they just don't realise it.
Plants, and fungi, are as evolved as any animal organism on this planet. Their bodies are different, they live in a totally different way and they interact with each other and their environment in such an alien manner that, as animals, humans simply don't notice.