It's also illegal to partake in cigarettes in certain areas, even for adults. Cigarette packaging and location is also regulated. It's not like adults have unfettered access to things that harm them. It's illegal to drink and drive. In some places, it's illegal to drink in public, or it's illegal to sell alcohol beyond certain time periods. I'm not saying that these things are right, but that if we're going to compare social media to harmful substances we shouldn't misrepresent the access to harmful substances adults actually have.