In the USA? I've never had an offer letter that made a statement like that, nor did I ever sign an offer letter. I interviewed for the job, got a verbal offer over the phone, received a written letter with one or two sentences confirming the start date and salary, and that's it!

EDIT: I'm referring to highly compensated professional jobs. In two such positions, I only received a verbal offer, and there was nothing in writing. Normally, I'd insist on something in writing, but in these cases, I was hired by someone I trusted.