I keep my word when I'm dealing with people. With people my word is bond.
With a company, all that matters from an ethical and legal point of view is what's written down on paper and signed by both parties.
If you sign a contract promising to begin work on a certain date and there's no penalty for changing your mind, then it's OK to change your mind. If they wanted a firmer commitment than that then they could write in a penalty clause to cover that circumstance. Of course, you'd be entitled to ask for more from them in return for this commitment (e.g. increased signing bonus).
You may let down a few people at the first company if you change your mind and take another offer, but if you're a manager at a company then you should be well versed in dealing with the mild disappointment arising from an individual choosing to act in their own interest rather than the interest of the company.