The motive of the Minds letting humans be in places or do stuff just because they think it's fun isn't uncommon. No Culture ship needs a crew to run things, but they'll have them anyways, listen to them, let them "help" build new ships, because they like doing so. But they don't have to and if it gets dangerous they'll not bother and just act, confident in their superiority. Minds are almost without exception perfectly nice and respectful about it (much more than we are to our pets, which is key to it being an Utopia), but the dynamic is nevertheless there.
> I am a Culture Mind. We are close to gods, and on the far side. ‘We are quicker; we live faster and more completely than you do, with so many more senses, such a greater store of memories and at such a fine level of detail. We die more slowly, and we die more completely, too. Never forget I have had the chance to compare and contrast the ways of dying.’
Gods that really love and respect their "subjects" and creators though, so they'll not do any of the terrible things they totally could do without any chance of stopping them, but rather support them and their whims as far as they think is safe.
SC is a bit special because non-Culture civs don't necessarily play well with AIs, and somewhat open how much Minds engineer the situations that occur. And in that SC is explained as somewhat of an outlet for those for that the utopia doesn't quite work - but it wouldn't exist if the Minds didn't think it was good to have.