They refused to comply with the police. Pepper spray is preferable to physical removal, less violence.
http://youtu.be/HaeCqChRQms
The kids were sitting in a line. I guess this was some military grade pepper spray or something and police regulation permitted use only when threatened with force and when 40 ft away. One kid had to spend the night in the hospital after suffering nerve damage from being zip tied so tight they couldn't cut him free without taking skin. This is excessive force by the police, and the fact that it was ordered by the chancelor of the university is pretty bad.