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.
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.