You're right... these notices are weird. They're also a scam.
Most counties who installed expensive red light camera systems now realize they can't successfully prosecute the cases, due to a recent US Supreme Court ruling on evidence, so they've started sending out these notices.
There is no legal authority for any action against you if you ignore it.
NOTE: Make sure the notice you received on a red light ticket isn't a notice to appear in court! If you ignore one of THOSE, you could wind up with a warrant for your arrest.
For more information, check out www.highwayrobbery.net
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.