I think another reason to trust a VPN provider is that it's their reason to exist. If people find out that they're not really protecting their customers' privacy, they'll lose all their customers.
ISPs, on the other hand, provide a different service and privacy is merely an additional consideration. It's not their main reason to exist.
Not even that. Look into CALEA - a law which, summarized briefly, requires that ISP gear have backdoors for law enforcement packet capture.
There's a reason you don't see anonymous ISP's around in the USA; the laws as written explicitly prevent them from existing in any meaningful capacity.
> If people find out that they're not really protecting their customers' privacy, they'll lose all their customers.
Yeah, but given the lack of history, openness, and reputation a lot of the VPN providers have, it seems to me that a provider could just "pick up and move": start a new company with a new name and new IPs and what not, and just do it all over again.
Not to mention that there is also a pretty big incentive to sell out your users. Probably a high percentage of shady business that the entertainment industry normally can't get a hold off legally.
ISPs, on the other hand, provide a different service and privacy is merely an additional consideration. It's not their main reason to exist.