Another way to look at it is to see what would happen if the contract was valid.
If they hire you and you simply don't show up, normally they won't be able to claim damages, but they will fire you pretty quickly. Depending on the country you're in and the excuses you come up with, they might still have to pay you up to a month's salary for not showing up. This is the worst case scenario for the company.
Because of this worst case scenario, a company will not enforce a contract unless the contract you signed includes no show penalties - but I can't think of any such example, except for contractors. Normally if you tell a company you want to annul an employment contract before it started, even if both parties already signed it, it's in the company's own best interest to let you go.
On the topic of professionalism: Looking out for your career is professional, although it's not always nice, and not always ethical. The people who wanted to hire you might be disappointed with your actions, and file you under "don't ever make an offer to this guy again". Anyone else won't hear about the story, and even if they do they most likely won't care.