I think that's an overly simplistic heuristic. We've (sadly) seen plenty of instances where care in the NHS wasn't great, whether it's in Staffordshire or Baby P. It's a huge system so it's inevitable that there will be things going wrong in it. That doesn't mean that they aren't putting standards first, just that humans are fallible.
Equally, I don't contend that private companies will automatically look to screw customers over at every chance they get. For instance, a lot of the healthcare start-ups I talk to are run by doctors. It's hard to imagine they're ethical by day, bastards by night

Most companies I've dealt with in this field just want to make a difference, and indeed many have taken to starting their own enterprise to do so because of the often sclerotic and cautious nature of things in the NHS. They simply think they can innovate and experiment more effectively outside it than inside it.