It is generally agreed that the United States can prohibit intrastate sales of goods it feels might affect interstate commerce under the Commerce Clause. How does intrastate gambling differ? Obviously gambling isn't a tangible good, but I would imagine it could affect the interstate demand for the service.
From the holdings of May 14, 2018 in NJ. v. NCAA,
As the Tenth Amendment confirms, all legislative power not conferred on Congress by the Constitution is reserved for the States. Absent from the list of conferred powers is the power to issue direct orders to the governments of the States.
The federal law in question forbids legislatures from passing certain laws, but
PASPA’s anti-authorization provision unequivocally dictates what a state legislature may and may not do. The distinction between compelling a State to enact legislation and prohibiting a State from enacting new laws is an empty one. The basic principle—that Congress cannot issue direct orders to state legislatures—applies in either event.
The anti-authorization provision does not
constitute a valid preemption provision. To preempt state law, it must satisfy two requirements. It must represent the exercise of a power conferred on Congress by the Constitution. And, since the Constitution “confers upon Congress the power to regulate individuals, not States,” New York, supra, at 177, it must be best read as one that regulates private actors. There is no way that the PASPA anti-authorization provision can be understood as a regulation of private actors. It does not confer any federal rights on private actors interested in conducting sports gambling operations or impose any federal restrictions on private actors.
The Commerce Clause does not enter into the majority opinion, though in Thomas' concurring opinion, he indicates he is not entirely happy with purely in-state applications of the Commerce Clause. But, at present, interstate gambling does not differ.
Commerce is the exchange of money, which is for any debt both public and private (such as a gambling debt). A similar tangible good need not be traded in exchange for the money to be in debt to someone. The Federal Government may regulate any debt that would cross state lines and they get quite nuanced about what can be considered a debt. Very few products in the U.S. originate in one state and are sold exclusively there, though they do exist.
So gambling services (casinos... sports gambling... ect) typically are selling the thrill of a risk of winning big money on gambling (some people like it. Personally, I'd rather keep money I already have, but hey, if it's your thing, you can do it). Gambling typically tends to be either brick and mortar based (Horse Racing, Casino's, ect.) or state run (Lottery) so the cash is considered in state at the beginning and end of the transaction.
Suppose I was a citizen of State A and traveled across the border to State B with $500. This is not commerce as the Federal Government can not regulate my personal movement across state lines. State B has legalized gambling and I bet all $500 dollars and win or lose a sum of money from that bet. This is commerce but the product was purchased and consumed in State B (I can personally take my own money across state lines.). If I win, then that transaction happens entirely in State B. Say I win big and get $1 million (And do the Dr. Evil pose to boot)... that money is given to me in State B and then I return home to State A with it without commerce as the transaction must conclude before I leave State B.