As a VPN Company Owner myself (torusvpn.com) -- I can really only speak for myself, our company, and what we believe.
You bring up a really good point. There's really a lot of trust involved between a VPN Provider and its customers.
In general stick to paid VPNs that are transparent and personal. From personal experience, I can tell you that a majority of VPN owners won't sell your data because they are against it. They created their VPN service because they believe in digital anonymity/privacy.
"But how would a VPN service (free or otherwise) even be held accountable for that, other than potentially consumers finding out and there being some big public scandal."
It's a tricky situation.