From what I understand, a lot of anger from cultural appropriation in general comes from considering parts of another people’s culture to be just neat costumes to wear without understanding or caring about their significance to said culture, especially since it sometimes comes with a healthy dose of ignorance/racism. For example, some Native Americans consider war bonnets as part of an “Indian costume” to be akin to “stolen valor”; the bonnet apparently serves a similar purpose.
I’ve seen someone say that white people seem to treat other cultures like Jack treats Christmas in A Nightmare Before Christmas: perhaps starting with good intentions, but clumsily not understanding, or thinking they have to understand, anything about what they’re aping actually means.
As for this particular situation, I do recall seeing some people assert that dreadlocks have a larger significance than “just” a hairstyle, but I don’t know the truth of that. Maybe they’re ticked that white people can wear them without a problem, while black people who have them are sneered at for being low class, and encouraged to straighten out their hair or they won’t be considered good looking. (Maybe double standards is a big sticking point here, now that I think about it.)