Working illustrators get paid to illustrate 'to brief' - so although the artistic flair and ideas come from the Illustrator, they are still working to a specific brief.
Now the fact that they are working to a brief ... does this mean it is not Illustration? I feel it's still illustration and still has it's artistic merit.
For instance the work I did for
Mums on Call I feel is artistic, but I did work to a brief to match business needs and use of the illustration.
I don't see how that piece of illustrative work could be called 'graphic design' rather than illustration simply because I was working towards a brief for a commercial client instead of working 'completely freely' to my own whims.
Illustration is illustration, whether one is working within some parameters or not.
I always describe myself as a 'graphic designer and illustrator'.
Amanda