What's the specific utility you see in a definition for pain that would exclude one or both of your examples? Historically, as in the article, such reasoning has been used to justify actions against an organism that would clearly be cruel if done to a peer. That can have some value; it's a decent answer for why it's okay to mow your lawn but not to mow your rabbit. You've got to be really, really careful, though, that you're not simply creating a post-hoc justification for the status quo.

The way I wrap my head around this problem, slightly simplified, is as follows: Rational beings have a duty to minimize the suffering they create. All suffering is a negative stimulus. Therefore, minimizing the negative stimuli I create is sufficient to fulfill my duty. Even though this means I'm refraining from certain negative stimuli that are debatably not suffering (squashing ants in my house, breaking a branch off a tree to play with my kids, etc.), that isn't a large cost to be certain I'm not unnecessarily creating suffering.