He claims that black people are less intelligent. The Times quotes him as saying
… you should not discriminate on the basis of colour, because “there are many people of colour who are very talented, but don’t promote them when they haven’t succeeded at the lower level”. He writes that “there is no firm reason to anticipate that the intellectual capacities of peoples geographically separated in their evolution should prove to have evolved identically. Our wanting to reserve equal powers of reason as some universal heritage of humanity will not be enough to make it so”.
So Watson doesn’t seem to have any “firm reason”, beyond than his own personal views about black people, to say what he says. The Science Museum commented that his views go “beyond the point of acceptable debate“.
It’s called a no platform policy. I think it’s fair enough.
It seems the Oxford Union, on the other hand, will give anybody a platform. I can’t comment on their motives.