If you realize “friendzoning” is just a modern name for unrequited love, it seems a lot less ignoble and more understandable.
You can’t be mad at someone for friendzoning you, that’s dumb and self-centered, but you can easily be mad at the situation of being friendzoned and that’s not at all unreasonable like some people make it out to be.
Misalignment of expectations in personal relationships is genuinely terrible and you’re not an asshole for not being able to be friends with someone any more because they don’t feel the same as you, not in my book at least.