The Mercenary in a White Coat

I've been thinking about this a lot, the "mercenary" aspect of medicine. 

Mercenary in the sense that I keep hearing people say they don't like a certain aspect of a field in medicine, but that they do it because it pays well. 

I find that a little unsettling.  How good is a doctor who does something they're very excited about and read all the literature on compared to someone who does it for the money? 

Are the docs for hire as diligent and interested in patient well being as those who practice because they genuinely like the field?

Are they both as ethical?  Do they both bill the same way?  

It's a disturbing thing I've come across.  Like the flash of a sinister shadow in the hallway, I know it's there, and I know I'll only see it again and again.