Wait, hitchhiking is illegal (in the US presumably)? (Supplemental question: how do you make hitching illegal?)
In the UK I've met many interesting people both while hitchhiking myself, and while picking up hitchers. It is a practice that seems to have almost entirely disappeared here, not because it's illegal, but I guess because most people now have cars and some "stranger danger" worries.
https://hitchwiki.org/en/United_States_of_America
As for "how": legislature passes a law against it like any other traffic law. Similar to jaywalking or prostitution (soliciting sex on the side of the road).
https://hitchwiki.org/en/United_States_of_America
As for "how": legislature passes a law against it like any other traffic law. Similar to jaywalking
You should watch Rambo: First Blood. Great movie.