Arguments that they are digital game items with no "real world value" is a very bad argument to be tried to use here. That same argument could easily be used for BitCoin, gold, or silver, and yet, those are all considered to have value. If you tried to make a casino that ONLY took one (or a combination of those) as payment, they'd still under the jurisdiction of the local gaming laws. I'll add the site is a raffle site and almost every state in the USA considers raffles "gambling". There are very specific requirements which can allow them to be operated but that site definitely doesn't follow any of them.
It doesn't matter who assigns the cash value to it, just that enough people assign cash value. That is all that matters and given how many users sites like OPSkins.com have (over one million users), I'd definitely have to say that these game items have enough people that consider them to have value.
Also, just because you use something that doesn't have value to an outside observer, doesn't mean it's not illegal. They could be using blades of grass for all that matters, it would still be under the jurisdiction of gaming laws if there is a major cash exchange for blades of grass. In reality is just means they're using a fiat currency and that makes sites like OPSkins.com money launderers.
The only reason the question wasn't raised earlier was because the only gambling site prior to CSGOJackpot.com was CSGOLounge.com which is located in eastern Europe. On top of that, it has no house cut and therefore it allows betting there to be classified as "a game of skill" since you can obtain an edge and consistently win. Though that doesn't mean it wouldn't still need a gaming license given Navada's recent decision to ban paid fantasy sports sites FanDuel and DraftKings from operating in Navada.
However, with the rise of many other gambling sites, expect the government to come in and start saying something, especially given that about half of the game's population is under 18.
EDIT: One last point I'll make is that to say the game company doesn't set any prices is also an invalid argument because Valve sets the prices for keys, stickers, and sticker capsules (which it sells all three directly to customers). These are all fixed prices in USD. While there are sales for the sticker stuff here and there, keys are always a fixed price of $2.49 USD. This never changes and therefore Valve pegs the "value" of a key to be at most $2.49 USD. Because they don't let you sell it back to them, you can sell them second hand at a lower rate but the basic rate is still based on the $2.49 initial sale price.
Also, given the current laws on digital properties, the Steam Community Market Terms of Service, as well as the IRS's rulings (which you can confirm with an IRS agent), it all comes down to the secondary transfers of items being viewed as being taxable. If it's taxable, the economic value is there and gaming laws will apply.