Also, can they hassle people about which state their car is registered in? Is it their job to report insurance and registration irregularities to the California DMV?

I'd like an answer from someone who has some knowledge of the law in regard to this, particularly in regard to crossing from Mexico into California.

Most border agents are polite and do their jobs. However, in my own personal experience and in the experience of friends and relatives of mine, I have noticed that some agents seem to have personal agendas motivated by their own insecurities and ego trips.
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I notice a lot of people did not pay attention to the phrase "from Mexico into California" in my question.

Also, I never have "acted like a jerk" when crossing the border, or given any customs or immigration official a reason to act that way. In the rare instances where they have been rude, it was totally unprovoked.

Yahoo Answers members may pick a best answer here, but I won't. There are some useful speculations here, but with the erosion of our human rights during the past 8 years, it's hard to know just what rights border agents have.

Suffice it to say that there is increased cooperation these days between federal employees such as immigration/customs agents; local and state police; and even the military--whether justified of not.

I sit now awaiting the results of a presidential election, hoping that we will have a new president that will work to reverse much of the destruction of our constitutional rights carried out by neofascists.