A lottery isn't necessarily inherently a scam, at least no more than gambling is in general. In practice the odds of winning are pretty poor compared to alternatives but as long as they're up front about the odds of winning I wouldn't call that a scam. Eg, this lottery lists the odds of winning each prize, though it would definitely be better if they published those on the main page rather than in the terms. A fairer lottery is possible pretty easily by adjusting the prize values, range of numbers to select, or how many numbers the gambler selects. This would kind of defeat the purpose of most lotteries to raise money for government, but personally I'm for more progressive taxes anyway.
Advertising one prize when the real prize is significantly lower is just lying and not an inherent trait of lotteries.
Okay, I'll simplify. Store advertises three sandwich. You buy three sandwich. You get one sandwich. Store says fees+taxes ate other sandwiches. You say it's fine, you got sandwich. I say it's not, store lied.
They absolutely can give a solid number even when a lottery runs in areas with different taxes, they simply choose not to because they make more money that way and for some reason you lack regulation there. See for instance here where the prize money may be partially subjected to income tax, meaning tax varies wildly depending on the winner's other income:
So there's three obvious choices: mislead customers, calculate the correct prize after relevant taxes and advertise that, or give a fixed value and eat the cost of any taxes themselves. They chose the first one.