It has a lot to do with how much power the "employer" has (one of the 5 factors discussed in the answers).. its pretty normal that someone can employ you today to contribute to opensource and choose your direction, or not employ you and hope your direction and theirs doesn't fork..

(When they try to control all the PMs, etc but accept public work when it happens to fit, then I do think they are in the grey area.)

edit-terms&redundancy..