Nor does the Culture force individuals or groups into joining, though it is not above meddling in the culture (heh) of other populations to make them (say) stop

Nor does the Culture force individuals or groups into joining, though it is not above meddling in the culture (heh) of other populations to make them (say) stop genociding each other. FWIW, Banks himself stated that exploring how a "perfect" utopia would deal with less enlightened neighbors is a core theme of the series. It's all good and well to provide perfect harmony for all your citizens, but can you truly be said to be "good" if you allow your neighboring civilizations to live in squalor? On the other hand, meddling in the affairs of others has many ethical quandaries as well. Exploring these is what the culture series is all about.