Being suspended or expelled, those people could not be Labour candidates in May, thereby potentially saving the party dozens of seats that such candidates would certainly have lost. The Teaching Assistants, being council employees, cannot be council candidates. But their supporters include many people who not only could be, but in several cases should have been years ago.
The NEC can grant membership and candidacy on the spot in emergencies, and this is an emergency. Alternatively, it can watch the devastation of the largest Labour Group in local government, taking the authority to No Overall Control while at least depriving the Leader of his seat before he lost his liberty. Of course, as supporters of Jeremy Corbyn, the majority of the NEC's members may actively wish to see that outcome.