This is pretty big news. It is somewhat reminiscent of when the US Department of Justice reversed its opinion on The Wire Act. Effectively, both the SCOTUS decision and the prior DOJ decision had the effect of turning over power to the states. In this case, it is online sports betting and in the prior DOJ case from 2011, it was online poker.

Now, effectively, states can decide for themselves if they wish to allow online sports betting to take place within their borders. Online poker hasn't had much luck since 2011, with only 4 states passing pro regulatory laws and only 3 of those being implemented thus far.

In 1992, Congress passed PASPA, which prevented states from authorizing or licensing sports-betting within their borders. Four states that had existing avenues for wagering on sports ? Nevada, Montana, Oregon, and Delaware ? were grandfathered in and allowed to continue to permit betting on sporting contests. Of these four states, only Nevada made much use of this exemption, and Las Vegas became the capital of U.S.-based sports-betting.

However, it is believed that sports gambling online will fare much better than poker. The reasons behind this is that it is:

a) much more profitable than poker on a pound for pound basis
b) more socially acceptable
c) entrenched casinos, with huge lobbying dollars, want it much more than they do poker

Some states are already poised to pounce:

Pennsylvania, Mississippi, and West Virginia already have statutes on the books allowing for legalized sports-betting with a catch: They were written to only become active in the event of the invalidation of PASPA. With the recent Supreme Court ruling, these clauses have been triggered, and so the three states can proceed with their implementations of regulated sports-betting ecosystems.
Another consideration is that of daily fantasy sports companies. DFS companies like DraftKings and FanDuel are drooling over the opportunity to add regular plane-Jane sports betting to their repertoires. And they too have huge centi-million dollar war chests to back it up with.

Whether this works out or ends up dragging on forever mired in petty bureaucracy like online poker remains to be seen. I bet the offshore gambling sites are sweating it hard though right now!