From Boing Boing:

Trey Muhlhauser Yesterday 09:37 PM
There is actually legal precedence in California saying pepper spraying non-violent protesters legally qualifies as excessive force. Case was Headwaters Forest Defense v. The County of Humboldt. The casual way the officer paused before spraying certainly implies he was not under specific duress. UC Davis Police department is going to get sued, and probably will lose.

Here is the find law article on Headwaters V Humboldt. Note that this shit has been upheld as illegal since at least 2002.