IIRC, to be considered a "mercenary" a group must be given independent command with clearly defined military objectives.
In other words, it has to be along the lines of "go take Hill 309 " not "go take those latrines to Hill 309"
Ever heard this before?: "Meet the new boss, same as the old boss"
Private contractors have a much more pleasant rhetorical ring to it than mercenary. Besides, mercenary has distinct negative connotations whereas private contractor is relatively new terminology and possesses far less negativity in connotation.
While PNCs can act as mercenaries such as Executive Outcomes actions in Angola and other African nations in the 1990s, I'm very reluctant to characterize their roles in Iraq and elsewhere under western nations directions as being in line with being mercenary.
Besides, people have this image of PNCs as being a bunch of lawless murderous thugs who hired on because they want to kill and terrorize without the oversight of a regular military. When in fact in areas like Africa where the aforementioned EO fought in the 1990s they fought very professionally and did a whole lot of good for the regions they were in.
Remember, during the horrific 1994 Rwandan genocide, Executive Outcomes offered to send their troops in (without prior payment) to help end the killing but they were turned down. It has been estimated they could've saved 200,000 lives.