The main point here is, they were doing a good job. If everyone had your mentality ("let's not hire this guy ever again because he did something wrong in the past, even if he was doing a good job now"), everyone that did something wrong in the past would remain unemployable, the end result being a collapsing society thus one would be forced to steal since you have to eat.
Signature advertising should not be someone's job. If this is someone's primary source of income, they are not someone we particularly want here.
I am not saying someone should absolutely, under no circumstances ever get hired to advertise on their signature again if they are caught plagiarizing. I am saying potential advertisers (employers) should be warned as to their history so they can take this information into consideration.
There are a decent amount of circumstances in which I would *not* personally hire someone who previously plagiarized, especially if they tried to cover it up after they were caught.