Pokemon creatures fight mainly for sport. That's why they enjoy battling with help of people. They don't want to kill their opponent. They just want to have fun battling.
The Pokemon that appear before trainers WANT to get caught. Humans and Pokemon form a symbiotic relationship. They can accomplish more together than they ever could alone.
Pokemon can only grow by fighting other Pokemon.
Pokemon in the wild eat each other.
A trainer can safely make Pokemon battle each other without one of the parties getting eaten or killed. A trainer can safely make his Pokemon grow more quickly, efficiently and stress-free in a loving environment.
A Pokemon that does not battle in the wild will pretty much never grow up. This is the equivalent to the environment having chemicals in it that suppress growth hormone. Battling in a safe environment is the equivalent to actually allowing a Pokemon to express these hormones and grow.
Also, most Pokemon psychologically *like* to battle. Remember the animu episode about the Snubbull that wanted to battle, but was instead pampered all day with different dresses? It got depressed because it couldn't exercise its natural need to battle, and grow older. Many Pokemon, like Iris' Axew, actually dream of evolving. Battling is the only way they can achieve this.
So ironically, in most cases, not letting your Pokemon battle is abusing them. In the other cases, leaving your Pokemon in the wild can kill them.
