So now museums and galleries have started collecting donated LEGO for him to use instead - in second hand cars.
A fair point is made that Lego are in a powerful position to make statements on a global scale, influencing the minds of children and parents, so they should perhaps be using that power for positive statements. From a personal point of view, I think that Lego are within their rights to treat with extreme caution the use of their products, as selling them in bulk to the artist may be seen as endorsing his work and opinions. They presumably have a lot of people request using their product in their work, and when aware of these projects, they have a choice - allow everyone to buy it and do what they like with it, or limit its use and try to restrict it to the sort of 'neutral' projects that won't impact on the brand negatively. Tough call, especially when your product is predominately associated with children.
Lego can't stop people donating Lego, nor Ai Weiwei making anything from it (as long as he stays within their guidelines about associating him with the project) so he's "decided to make a new work to defend freedom of speech and "political art".
Ai Weiwei's Instagram feed has become a fun account of people filling the donation vehicles via their sunroofs.
Find out about the Royal Academy donation point here.