Yes, it can be a bit confusing. I don't know the precise criteria used by the Chip Guide, but here's what I consider --

For the 48 contiguous United States, each state with casinos has permitting legislation that regulates when & where casinos operate.

Nevada is the oldest & broadest, permitting full casinos statewide (subject to zoning, and also a 200-room minimum hotel, though older properties without rooms are grandfathered. There's a different standard for slots-only places without table games. And there's one Indian casino, south of Laughlin, that operates under Indian Gaming rules.

Next are the limited-location land casinos, frequently in historic and/or tourist locations. Atlantic City, Deadwood and the 3 mining-town locales in Colorado fit this description. The land casinos in Detroit fit this model too, though done for economic development reasons.

Then there are the Riverboats, which are on or near navigable rivers in the USA. Mississippi, Louisiana (with one New Orleans land-casino added in their legislation), Missouri, Illinois and others fit this model.

Then there are Indian / Native American casinos, spread throughout the US, including some of the states above. They must be on Reservation land, and follow federal laws which define how they operate (including type of games, not to exceed limits of the state they are located within ... gets a bit confusing.)

In each case, casinos get approval from legislatures or voters, and usually follow the "little bit pregnant" rule (my term). As with alcohol or strip clubs, the state or locality likes the revenue, and the public likes the entertainment, but casinos are still considered "vices" in most places, so they limit where they can operate.

Over time, regulations tend to relax, like the riverboats on the Mississippi that had to cruise in 2-hour intervals, or the barges that floated in an enclosed coffer dam next to a river or Gulf Coast. Or the casinos approved for low limits only ($5 per hand max). After awhile, logic ... or disasters, in the case of Gulf Coast casinos ... prevails, and casinos migrate to land and the maximum bets increase.

BTW, if you want a concise history of each states' legislative history, the American Casino Guide book (the book published annually, with coupons in it) has a pretty good summary at the beginning of each states' listings.