You have to understand, much of he law that pertains to using a motor vehicle in the US is a pretense to legitimize traffic stops, which are then used for some other purpose (mostly busting drivers for more serious crimes, or else extracting fines from out-of-towners/minorities).
What is the point of having traffic stops of this kind? For example, why can't you just have traffic checks as a normal activity? In some countries (Botswana is an interesting example) the police stop you on public holidays and give you flyers and tell you "drive safely".
Our system allows police to selectively apply traffic rules. This means that the people who find them onerous and can fight back against them tend to be avoided, while more vulnerable people are subject to them more often.
You might remember the protests in Ferguson, MO, after Michael Brown was m̶u̶r̶d̶e̶r̶e̶d̶ killed. A federal investigation later revealed the tense police/public relations under which that incident took place: the jurisdiction was essentially using the traffic laws to extract rent, overwhelmingly from black residents. A traffic citation would require attending court (often without access to a vehicle); if a court date was missed, additional fines and an arrest warrant were issued, which of course the person would only become aware of during the next traffic stop. Police were encouraged to increase stops and citations to make up for lowered taxes. It's regressive fiscal policy you see popular among American conservatives because it shifts the tax burden off of people who are then more likely to vote for them, onto people "deserving" of punishment.
The protection from unreasonable search and seizure is taken to mean that the police can't stop your car without a reason. Since the justice system really really wants to be able to stop your car at will, anything at all can be a "reason".
That would violate our constitution. These sorts of traffic stops are either a bug or a feature, depending who you talk to. I feel that in the absence of clearly reckless driving the police should be unable to stop you. I view these sorts of stops as government run amuck, and a police state. Many Americans would agree, which is why we have constitutional protections beyond most countries (in theory at least). The problem is that hinders the police's ability to solve cases, which is where the "hack" comes in.
Yea...US DUI laws are "interesting". I make every effort to not drink and drive, but I definitely don't let anyone in a car I'm driving have an open drink. I've seen way to many 'you just handed your drink to the guy in the back seat' scenes to risk that.
Not everywhere, and it should be nowhere. Which states don't specify that there needs to be alcohol in the container? That seems like a bizarre last, but it certainly wouldn't surprise me.
A couple of German exchange students once told me that you can drink in a car while the car is moving as long as the driver doesn't drink anything.