GameOgre
Adventurer
LOL! OK, bud but in what way is the answer I've been trying to get at. Again, I'm not saying you are wrong but you still haven't said why these examples are so different when, to me at least, they seem similar in that you have a single person deciding unilaterally to effect change that has an impact on all involved. Why is the PC's reason for changing characters more valid than the DM's reason for bringing in a new character?
Because of the Unwritten laws of first picksies! Once you pick a job in the group nobody else is allowed to pick it. EVERYONE knows that.