My younger self would have said the same thing, that you have a moral obligation once you make a commitment. But having been around quite a while now, I have seen companies retract offers between when it was accepted and when the person was supposed to start, because they "changed their mind", or had a change in circumstances. So I take the opinion now that as long as you are up front and honest it is just a business transaction and the karma balances out by being up front.
"I have seen companies retract offers between when it was accepted and when the person was supposed to start..."
I've also seen companies hire people and then turn around and fire them after a couple of weeks because they "weren't working out" or because "the funding for the job disappeared" or because they had layoffs due to financial difficulties (which they probably could have seen coming at the time they hired the person).
"At-will" employment goes both ways. If the company has the right to fire an employee because it's convenient for them (or just because they feel like it), an employee shouldn't feel bad about firing a company for the same reason.
