Why is there a minimum Ic for a optocoupler?
The stated Collector Current rating in the datasheet refers to minimum and maximum guaranteed current that the transistor collector will pass, for a current of 5 mA through the LED. The variation accounts for manufacturing differences between pieces, not for an individual part.

This does not prevent the application from passing less current by external limiting of the current.

The datasheet information, and the corresponding behavior, are similar to conventional BJTs, other than that the corresponding base current is not specified.