Android - Why can't I change the Google Calendar on an event after it's been sync'd?

This is probably because of how the Google Calendar server API interacts with client applications.

An update request has to be sent to a specific calendar on the server. If you could change which calendar the appointment is tied to, synchronization would have to send a "delete" from one calendar and an "add" to the other.

I've developed and maintained a number of systems which have synchronization and replication... and I suspect that a decision as made to favor simplicity and avoid the possible complexities of cross-calendar edits.