Yes, dash cams are legal in every state in the US. However, 36 US states do not allow a dash cam to obstruct the view through the windshield. These states are: Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Idaho, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Mississippi, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, New York, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Virginia, Washington, West Virginia, Wisconsin, and Wyoming.
That's a long list, but as long as truckers mount their dash cams between their rear view mirror and windshield, it will be legal anywhere in the US, without cause for concern.