Sonos will pay for you to ship your recycled speaker back to them to be recycled. Not only do they pay for this shipping and recycle the speaker, but they give you a 30% credit. Hardly zero cost to Sonos, and considering the thriving market for refurbished speakers from Sonos it seems pretty clear the old speakers are being reused. Overall, sounds like they're doing exactly what they say they are.
They don’t accept the old speakers (let alone pay for shipping), they ask you to bring them to a non-affiliated recycling center. They are never reactivated.