It's not so simple. Normally people, at least in cities, rarely hitchhike or take up hitchhikers. The goal here is to make it acceptable, and naturally greatly increase the frequency this is happening. This can be reasonably taken as an endorsement by Google to the driver/passenger, because it is -- they want this to happen more often, and are essentially saying "it's OK to hitchhike through our app". This means that they can reasonably be seen as taking part in the entire transaction and assuming some portion of the liability.