Just wondering here, i'm not years in the AM business but if you put some critical/"the selfish gene" thought paradigm here: Being opportunistic beings in such a fast paced make it or break startup market, + the pressure from VC/investors to produce revenue + who doesn't want to make a million dollars with his startup, have parents be proud + all those women that never paid any attention to you now think your cockalicious, etc. But with all seriousness now:
A traffic provider has everything they need to copy-paste your hard earned 170% ROI campaign created (with 100% risk on you) Not only do they have every bit of your campaign information, from the banners to the targeting and publishers, they also have better buying power and reach than you as a DSP/Network... I'm not talking about google or facebook, I'm talking more about those Affiliate-Dude-Just-Got-VC-Funding-DSPs.
So maybe they won't come with $200K and overbid you and send you home, i'm talking about just a constant sip off the 0% risk tit.
They can do all this without you ever knowing.
What do you guys think?
I think that if someone will do that to you, he wont last long in the industry. So I wouldnt worry about it too much.
Yes, there is always that risk, but if you work with known providers, you can be pretty calm.
Try to find info on them, see if they do AM in-house as well...
It's always a risk I guess, and you hear rumours about companies doing this, but like you said it can be hard to tell or prove anything.
I like to really get to know sources I work with and establish a good personal relationship with them - I hope that that way I can judge their character a bit better.
At the end of the day though, if you don't trust anyone... you won't go very far (unless you go start up your own private DSP or something...)
I hear that stuff like that does happen - both with traffic sources and affiliate networks.
However, instead of letting that discourage me, I put 100% of my focus on doing the best I can to maximize my own earnings.
Same goes for other sh*t that I know is going on in this world. Why concern myself with what I can't control? Especially when there are so many positive aspects to so many wonderful things and people that I could put my attention on instead! 
So my advice would be to just do the best you can to get your share of the pie - there's so much money flowing everywhere, there's more than enough for everybody.
Amy
For Affiliate-Dude-Just-Got-VC-Funding-DSPs to steal a one time campaign that makes 200k profit is a very short term view indeed. Likewise for some people it isn't just simply how much money is made how quickly but a long term view towards personal development, pride in achievement, and giving back.
Just my two cents.