Who gets to decide where to draw the line?

Well when I grew up in the early days of the internet, the plan was to have it decided by a democratically elected government that acts in good faith.

Since that didn't seem to work I'm kinda out of ideas as well

Setting aside the bad analogy, real people are much more likely to eat broccoli than to do heroin.

Once they've had a taste of both?

I am confident that the number of broccoli eaters exceeds the number of heroin users.

I would posit that is because many more people have had a taste of broccoli than of heroin.

You would posit based on what? Defunct rats-pressing-pleasure-button studies?

Many people have had "heroin" in the form of morphine in the hospital. They don’t turn into morphine-seekers.

Addiction and pleasure seeking is more complex than having a taste and then being hooked forever. Of course many people can't get out of this Skinner Box type of mindset.