The government prevents people from engaging in activities even when they do not violate others’ rights. A few examples are drugs, prostitution, gay marriage, kidney-selling and gambling.
Notice that nearly all governments in the world have such paternalistic policies. Notice that nearly all such governments allow emigration to other countries that do not ban drugs, prostitution, gambling, etc. That leaves us with a mystery to solve. Why would the government allow people to leave the country if doing so allowed them to engage in all these sinful activities?
You might say that preventing emigration would be too intrusive into the person’s life. But the government is already intruding into their lives by putting them in jail for doing some of these activities (namely drugs), so it’s hard to believe that it’s freedom we’re worried about.
I suspect that the motivation behind these laws is not a desire to help people but rather a desire to signal the society’s values. If an American wants to use drugs in Amsterdam, that doesn’t bother us so much since we’re not really worried about the drug user but the shame the drug user brings on the rest of us. To legalize drugs, prostitution or gay marriage here would signal that our society approves of such behavior, and we don’t want to think that about our society, and especially ourselves.