You sign an offer letter, not a contract. Its basically a promise that if you show up on a given date that your employer will give you the necessary documents to sign to start employment, and what the terms of employment would look like. There is no legal protection for candidates who have had offers rescinded. Employment in the US is 'at will'. You can leave with zero days notice, your employer can fire you with zero days notice.
A letter is something that is signed by the sender, not the recipient. The sender here is presumably the (prospective) employer, not the employee (to be).
So if (the editorial) "you" also sign(s) it, it isn't really a "letter" AFAICS.
[Edit:] The common-language, and I'd assume in most jurisdictions also the legal, understanding of what "a contract" means is something like "a document signed by two parties wherein they both commit to certain obligations vis-a-vis each other". So all in all, I'm getting the feeling that this "letter" actually, legally, is some kind of contract; only employers don't want employees to realise that, so they call it "a letter". /[Edit]
> ...if you show up on a given date that your employer will give you the necessary documents to sign to start employment, and what the terms of employment would look like.
So what are those "documents", then, if not what in most languages would be called "a contract"?
Sign what? This thread has been full of "most jobs in the USA don't come with any employment contract at all", so what is that you're signing?