Confidentiality agreements are relatively common, but outside of that there is no written contract. Generally speaking, US jobs fall under a legal category here called "at-will" employment, which makes it easy for either party to end the relationship at any time. Personally, I've worked a variety of jobs and never signed a written contract for any of them. In a few, I've not received a formal offer letter.

Office jobs often do have an NDA, but otherwise a lot of trades and hourly positions don't opt for one. The retaliation for leaking sensitive info is either suing for leaking trade secrets (which is rare), or just firing them on the spot.