You say 'not allowed'.
Who precisely is preventing you from doing so? Your objection seems to be more that people disagree with you, since there is absolutely no barrier to you either making your feelings known to those charities or setting up your own organisations that match your own beliefs.
That is a long way from 'not allowed'.
But of course your BLM example is massively flawed - what you're describing is organisations or movements giving consideration to parts of the movement that might need particular inclusion, support or consideration. It is perfectly legitimate for black trans people to work within the BLM movement to try to ensure that the specific concerns of black trans people are heard and reflected in the movement. That's not forced teaming - black trans people are part of the black community, and they are part of the trans community. There is no forced teaming involved.
To be an example of 'forced teaming', it would have to involve, for example, white trans people insisting that the BLM movement should advocate for them. Which is not what has happened.
Similarly, autism charities choosing a particular approach to supporting trans autistic people isn't forced teaming. Because the people they are supporting are already 'on the team' so to speak.
To be an example of forced teaming, it would have to involve autism charities being compelled to provide support to neurotypical trans people.