I'd contend that this is still a rejection of a person... as the target of affection has decided the other person is not what they want as a romantic partner because...reasons. Being rejected is firmly independent of the reasons for the rejection. You may feel you would be a bad partner to that person... but that does not in anyway negate or mitigate that it is still a rejection.
"Some turn to hate" - Right...but that is a whole separate issue dealing with psychological imbalances rather than confusing the term "friendzone" with plain old rejection. We could have a entire separate conversation about that topic alone... though its not really pertinent to the current discussion.
"the guy who was told that she didn't share his feelings, just kept going" - Another separate issue. One that involves a lot of retraining society in what is socially acceptable and what isn't... and maybe even some counseling.
"As with most things, you can't control the other person's response" - Very true. Though we, as a society, can improve the likelihood that undesirable occurrences happen less through helping address rejection itself. Our society right now is designed in such a way where rejection is considered a truly evil thing to be avoided at all costs. Advertising tells us daily that we MUST be good enough so we don't get rejected... and the way to do so we must have the newest clothes, accessories, makeup, appliances, cars and so on. Its reinforced through our education... the whole point of an education, or so we're often told, is to GET that job... or put another way, so we wont get rejected from the jobs we apply to.
Instead, I think, teaching our young folk that rejection is not a devaluing event, but rather an opportunity to evolve and grow may be the way to go.
Mike or Lisa didn't want to date you? Hey, don't sweat it. You're still a good person! And now you have more time to work on your hobbies! - Something along the spirit of this statement I think.
Either way, it seems to me that casting it as the "friendzone" distracts from underlying issue that is rejection.
But that just my thoughts on it.