Because Zimbra only supports Linux, is designed for large enterprises, and requires special client-side programs to be installed just to do things like calendar sync.
It is fine if you control the whole ecosystem, but ironically in most scenarios where that is the case you're running Windows Active Directory anyway so therefore might have well pay a few pennies for Exchange CALs.
It is fine if you control the whole ecosystem, but ironically in most scenarios where that is the case you're running Windows Active Directory anyway so therefore might have well pay a few pennies for Exchange CALs.