I did in the 1970's, and it's perfectly legal (roulette, not slots):
https://www.ebay.com/p/Las-Vegas-Roulette-Intellivision-1980...
That you are never forced to pay is a defining aspect of “freemium”. You are always given the choice to wait out the timer/watch an ad for your next upgrade/level/continue/bonus/lootbox, rather than pay.
In the US you can't build a slot machine that accepts money and dispenses tokens that have value. That's often how things operate in Japan, but the same model is generally illegal in the US.
It's questionable if spending real money on virtual currency to buy "loot boxes" that randomly give things of value is legal. And if that's legal, it's not obvious if changing the rate of rewards is legal.
EX: People buying WoW gold.
IIRC, Blizzard doesn't profit from that directly and explicitly prohibits it in their ToS.
I could go on, but saying it's virtual is not necessarily enough to protect these companies.