I was recently at a job interview where I was informed that, if hired, I would
I share your view that such a clause is at best very shoddy. Not being a lawyer, I wonder if the clause is even enforceable; you might ask someone who'd know. It might be unconscionable (a term I wish lawyers applied to more things than they do!) and hence unenforceable, or it might be unenforceable because too difficult in practice for a court to enforce (say, by issuing an injunction forbidding you from accepting a new job!).
This may be advice of little value to a desperate job-seeker, but I'd steer clear of firms whose offers come burdened by such clauses.