I think there is a pretty good understanding on this topic, but questions still come up once in a while in VI Java API community and my inbox. Some people got errors while trying to manage free ESXi servers via API. The API support of free ESXi is limited to ready only according to a VMware KB article:
vCLI, PowerCLI, and vSphere SDk for Perl are limited to read-only access for the free vSphere Hypervisor edition. To enable full functionality of vCLI on a VMware ESXi host, the host must be licensed with vSphere Essentials, vSphere Essential Plus, vSphere Standard, vSphere Advanced, vSphere Enterprise, or vSphere Enterprise Plus.