The difference being that with plants, you really don't have much of a choice. You have to eat something. Stalking deer, on the other hand, is definitely optional.
Plants give off a pulse when harmed and communicate with each other chemically to inform other plants in the area of insect infestations so they can try to chemically protect themselves. Just because your senses fail to detect the ways in which they express their suffering doesn't mean they are any less able to feel and express themselves than the creatures whose self expression is more readily understood by humans.