If a restaurant has a sign outside that says smoking is permitted, then anyone who enters willing risks any dangers associated with smoking.
Any employee who works there also has voluntarily subjected themselves to it.
Business owners should be able to reserve the right to allow legal practices on their premises, as long as they provide ample warning.
For example, can you sue a business if you slip on a wet floor and injure yourself? Yes.
Can you sue a business if you slip on a wet floor and injure yourself, and they have a wet floor sign? No.
Again, you wouldn't know this at age 15 but there's no such thing as a restaurant with a "smoking permitted" sign. There's only the opposite, but of course only in some cases -- it's either no smoking on the premises or it's left unsigned and you have to roll the dice (or ask). But no restaurant is going to deliberately alienate three-quarters of its potential customer base with a sign telling them "don't bother coming in".
I'm sorry, but this was all I got from your post:
"You're a stupid kid with no business debating this. If a business put a sign up, they'd risk alienating people who would already be alienated if they ever went inside."
I wouldn't know if you're "stupid" but at 15 you don't have the experience to know that there's no such thing as a "yes smoking" sign, which is what you suggested. Nobody does that. And nobody
would do that, and I told you why. So your hypothesis won't work because there's no such thing.
Further, you can't "already be inside" if you're just arriving at the door, according to linear time.