As Vanderark says, the difference between Celtic and Arbroath is that one is a direct rival, one isn't.
And yes, he's allowed free time. However spending his free time supporting a rival is unacceptable. McGinn has a duty, in the same way that any professional person does, to conduct himself professionally and maintain the best interests of his employer outside work. If I made an arse of myself outside work I'd expect to be sacked by my employer.
Now, I don't want McGinn sacked but a boot in the baws and a word in the ear are definitely required.