Except that taking the knee does not imply direct support for the BLM organisation anymore. The meaning of the symbol has broadened way beyond the organisation and now is broadly understood to be an act in support of minority (mainly black) rights in general.
Thats the thing on one side you've got this symbol popularised by BLM and then claims they want to use it not in support of BLM but just to symbolise solidarity against racism then you have fans who see it as strongly associated with BLM and booing it because of that.
Question, if the players had decided to adopt another symbol to differentiate their protest from BLM as a political organisation, but to still show support to the civil rights movement, do you think the same minority of fans in stadiums would still be booing?
For quite a few of them probably not, that's the reason lots of them are booing, because of the association with BLM.