It's open source (https://github.com/zerotier) but not Free Software.

According to their pricing page,

> ZeroTier’s software is open source and free to use for most purposes including personal use, internal use within a business or academic institution, and evaluation for uses that require commercial licensing.

I was able to find this: https://github.com/zerotier/ZeroTierOne

That sentence is an contradiction, it's either open source or requires a commercial license, cannot be both.