The offer letter doesn't make a difference here. Having it in email is as good (in the US) as an offer letter.

The situation described here is a legally-binding offer made to a prospective employee, which was then accepted (also legally-binding). When they revoked their offer s/he could have sued.

Unfortunately, you can't go around suing people for reneging on offers, at least not in the US. It's expensive, and it gives you a bad reputation.

The fact that this happened, however, means that the company had no integrity and that the person who received the offer is better off working elsewhere.