I think some of that is just respect. It also separates him from the other heroes.
So far, he's never forcefully taken a skill from someone unless they were on the opposing team. Minase (sp?) was on the opposing team when he absorbed her skill. In that conflict, he only had time to get a couple of skills.
When they are on his side or at the very least in a truce, he waits for an invitation, even though it does no serious harm.