Though I'm not quite up on my California laws, I'm pretty sure popping a wheelie is against them. Something about reckless riding, endangerment, and stunting comes to mind, though I can't quite put my finger on it.
That said, those laws don't always stop people from popping the clutch and sending their front wheel to 12 o'clock. Maybe it should, however, as not everyone knows how to pop a wheelie and control it.
If a wheelie goes bad, it usually goes bad in a very serious way. So you'd think that the people that were tasked with enforcing said laws—the ones who'll give you a ticket, impound your motorcycle, and potentially arrest you for the act—would know better than to not do so near a bunch of seated people.
Yet, as I've mentioned in previous posts, most people are dumb. And that apparently includes cops, too. One of whom is likely to get their motorcycle license taken away from them as, at a parade in Palm Springs, California, they attempted a wheelie, only to crash into the parade's crowd and injure a bunch of them.