The difference between a "nice guy" and a nice guy is the false sense of entitlement associated with the former. A nice guy is nice because he genuinely cares about how other people feel, and sincerely wants them to be happy. A "nice guy" pretends to be nice, because he's been misled all his life by romantic comedies into believing that any person he's nice to should love him just because he's nice.
Well, you can say I haven't done anything wrong all you want, I'm still treated that way, and it doesn't negate the original assertion that people are treated as if they're wrong in these situations. As a thirty year-old single guy, I can attest that the amount of gender-based difference in how peoples' expressions are treated is immense. I have never seen a 25+ year-old woman who complains that she is single or unwed told that she's not entitled to love, because the assumption is not made that she feels such; only with men is this assumption made, that being disgruntled with one's singleness is somehow an expression of entitlement to love, or in some cases even sex. I have similarly never heard a woman who complains about having been rejected frequently told that she should consider changing something about herself, or that there must be something wrong with her personality. However, whenever a guy has been rejected frequently, the first "helpful advice" is that maybe he should change his personality, grow confidence, stop looking for a relationship and look for a friend, etc. I mean, I posted a facebook status lamenting being stood up for a date cold with no call and got told that I shouldn't feel "entitled" to a woman's attention. Something is decidedly wrong with that.
I understand fully the distinction you're trying to make, what I think is lost though is how utterly impossible it is to tell the two apart, and both how ridiculously bad at it most people are, as well as how overconfident they are about their ability to discern between the two. The fact is, these mindsets of the "nice guy" aren't likely to manifest in romantically unsuccessful people; you don't start expecting reciprocity of affection out of the blue, it's a learned mentality. I mean, is it any wonder that this recent guy was rich, and quite romantically successful, but had a vendetta against girls who rejected him? We guys who have faced rejection our whole lives, we hurt from it sure, but we're used to it. You don't get this kind of reaction from someone without them having a preexisting expectation of reciprocity. In short, not only are the guys who actually get girls' attention more likely to be this, getting girls's attention is more likely to make guys become this.
But rather than see this, we see this kind of mindset as generated from rejection, and from a hatred that comes with it. Thus, we have a situation right now wherein simply voicing your discontent with your romantic situation gets you labeled this and denigrated. And if you disagree, ask yourself this; my second paragraph had little relevance to the topic at hand. Had I posted an argument such as that to, say, /r/relationships, would you not think they would deem me one of these "nice guys"? And regardless of whether they're right or not, do they really have enough information about me to make such an assertion?