I've had a Benchmade 51 with a relatively uneven grind. This was on an unmodified out of the box knife. It was also not as sharp out of the box as my much cheaper Kershaw. I forgot to take pics of the grind though and it is not longer in my hands. There is also a thread open about someones 710 m390 with bad blade issues that Benchmade further messed up during warranty and resolved that issue with the customer only after he complained about the unsatisfactory warranty results (so I guess at least Benchmade warranty isn't all that bad - in fact I have only had good results from them). Anyway, Benchmade is a mass producer. So there will be mistakes here and there but they try to fix it. No mass producer can avoid that.
I think the problem is similar to what Polecat said. People expect better fit and finish. Except I think that it is not people expecting higher finishes than the price entails like wanting a sebenza finish on a griptilian, but rather that companies like Kershaw can give you sharper, just as well made knives for cheaper (not talking about materials such as handles or blade steels, just fit and finish). Now I'm not being a proponent for Sanrenmu or anything, but apparently they are a company that, for only a few dollars, a fraction of what any Benchmade will cost you, can provide the user with excellent fit and finish. So sure, when people buy more expensive USA made knives they can and do expect a solid product. I too would be disappointed if my limited edition bm 710 m390 had blade issues. Knives NIB over $100 shouldn't have to have such problems like uneven grinds if other companies can provide knives for $10-$20 without those problems. Why would I want to buy a $100 knife, then have to spend more money out of my pocket to send it in to get fixed and tuned up to the way it should have come out of the box as well as wait a week+ turn around? I shouldn't. Why would I need to personally sharpen a $50 grip if a Sanrenmu comes shaving sharp out of the box for only $8? I shouldn't. Yes, sometimes a few slip through QC. But I can see why some people are a little disappointed. Maybe the people who are picky and unlucky just happen to be the knife enthusiasts on BF. Maybe there are more cases that people don't notice or do notice but aren't part of BF. I don't know.