Being able to do sticky posts isn't a moderator ability. It's a forum ability.
Mod things are to do with moderation. ie control of post content and location (edit, delete, move, split). Being able to sticky is something you can control on a per forum basis. By doing it like that you can split up the ability of people to control what's going on with those who have a slightly more important role of controlling how things are displayed.
I'll admit some of the permission stuff is a little complicated and perhaps overly so in some places but it's all to do with variety and customisation of what you want to do. However, the basic presets that are available cover the majority of what users will need. It's just a question of getting your head around the difference between forum permissions and group/user permissions.
Brf wrote a KB article if you want to read more
Knowledge Base - phpBB3 Permissions