Did you promptly inform them about the light (i.e., before you departed from the dealership's premises)? As you said, the automobile was functioning when you activated it.
I would be interested to know the number of recorded occurrences of the defect and if they were aware of the Check Engine Light being illuminated before returning the keys to you (which they would have been, by the way, if the light was on as soon as you started the vehicle).
This topic is inundated with similar problems like yours, when a recall is initiated yet the situation worsens upon resolution. Halliwell Jones failed to correctly install my airbags during the recall, necessitating BMW's intervention to enforce a reinstallation.