You missed my point. Unlike any other business, there are specific laws on the books to regulate gambling. It is a regulated industry, and cannot be reckoned as comparable to unregulated business. It is not a simple matter of "buyer beware".
The existence of those specific laws on the books could render a state legally liable if it fails to enforce those regulations within it's domain.
Likewise, there is a matter of equity- If the local ambling joints and dens of sin ;) are forced to comply, but internet sites are allowed to slide, that could be construed as an unfair advantage given to the internet based site, making the state liable again for failing to provide equal protections under the law.
How is regulating the gambling industry substantially different than regulating, for example, the insurance industry? The states manage to write and enforce their own regulations about that without the federal government.