I've always had great, pleasant conversations with my CBP officers (is that the correct term?). I remember having a long chat with one of them (there was no line) about Google Books and how his wife loved ebooks. Another talked about his Android phone. Didn't start out with that of course, but usually once they realised I worked for Google it was off to the races. Being a native English speaker (Brit) and entering via SFO probably had huge amounts to do with it as well -- just wanted to point out that sometimes they can be pretty nice people and you can have a friendly conversation!
(On the flip-side my husband was very nearly deported because the border agent at LAX found my Safeway card in his wallet when he was re-entering on a visa waiver. Agent told him if he'd come along later in the day it would have been over, but he was the first borderline case and seemed like a nice guy.)
I'd say it's probably the same reason as "don't talk to the police". There's very few things that you can tell to make you better (usually those will be asked from you anyway - like where you're working, where you live, where you're going, etc. - and even there precise concise answers probably better than vague verbose ones) but a real lot of things that you can tell to make you worse, even without realizing it.
A Safeway card is a loyalty card from a US grocery store. It's the kind of thing you have in your wallet if you live in the United States. If you're coming over on the visa waiver, you're explicitly saying "Don't worry, I am not an immigration risk, I'm just going to be here less than 90 days for business or pleasure and then go back to my permanent residence abroad. Establish a life for myself in America? No way, I have absolutely no interest in that."
There are a variety of easy ways for a CBP agent to have your wallet in his hands. One is giving it to him to show an ID, when you hand over your passport. As mentioned previously, that is not the required input for the state machine, so don't offer it.
Yeah, he wasn't actually foolish enough to hand it to them right at the booth. He was taken aside for further questioning since he had already been in the US on a visa waiver within the same calendar year, which pattern-matched border dodging. He was forced* to hand over his wallet early on and the card was clearly there. There's obviously more context, but it's funny that such a minor thing came to be so important -- and sticks in my head 3 years (and one marriage) later.
* Not sure where this fell on the spectrum of 'force', but had he refused, that would presumably have increased suspicion. Damned if you do, etc.
(On the flip-side my husband was very nearly deported because the border agent at LAX found my Safeway card in his wallet when he was re-entering on a visa waiver. Agent told him if he'd come along later in the day it would have been over, but he was the first borderline case and seemed like a nice guy.)