I voted for yes, but I disagree that it is Immoral.
I've done this in the past - The conversation goes, I'm sorry but I've had a much better offer from another client. I will be working for them full time after my notice is up.
I then offer to work for them out of hours, at home and at weekends. Some clients go for that, some don't. I can't see the non-morality of exercising contract terms and offering alternatives.
I would also say I seen more contracts tested to destruction by clients and agencies than I'll ever do. Many many examples.