It is based on Tokyo Cabinet (which is under LGPL).

On the homepage of Tokyo Cabinet, it says:

"BTW, do you know Kyoto Cabinet? Actually, it is more powerful and convenient library than Tokyo Cabinet. At this distance of time, Kyoto Cabinet surpasses Tokyo Cabinet in every aspects. I strongly recommend you to use Kyoto Cabinet."

I wonder why that isn't used. Maybe because Kyoto Cabinet is under GPL.