The legal issue is the presence of consideration, not its quality. You were still required to license that stick figure to Anthrocon. It was this licensing that was the consideration, not the act of publication. An entry without consideration was invalid:
If we do not have a completed release form on file by the deadline, we cannot enter you in the drawing or use your submission(s).
A lottery may require $1 to enter, for a prize of millions. I know plenty of furries who sell stick figures for $1.
The legal issue is the presence of consideration, not its quality. You were still required to license that stick figure to Anthrocon. It was this licensing that was the consideration, not the act of publication. An entry without consideration was invalid:
A lottery may require $1 to enter, for a prize of millions. I know plenty of furries who sell stick figures for $1.