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Because if there is money involved there is incentive to do it for a living, which leads to incentive to cut costs and that might not always be done in a consumer-friendly way; therefore regulation is needed. If there is no money involved, the people who would cut costs and make it unsafe for the consumer are rarer -- since they can't do it for a living, and the relationship is harder to regulate since the only thing involved is communication between two parties, so it's probably not worth it.

Because in the donated service (car pooling) its much less likely that the people participating are doing so as members of the public.

Its like the difference between me showing a movie in my house to 15 friends And me renting out a movie theatre and opening the doors for everyone to come in. Even if the same 15 friends show up, they do it as members of the public.

I'm pretty sure he sees the point you're making, but disagrees with it. While your question, which is already answered in the post you're replying to, indicates to me that you're missing the point.