Is there some overarching philosophy to the motivation of US laws, especially recent ones?
To what extent do laws describe customs already established in society, and to what extent are they implemented to change some behavior of society? What are the points of view on this currently dominant among US judges, prosecutors and attorneys?
Like most common law jurisdictions there are 3 sources of US law: legislative, judicial and administrative (except Louisiana, it has a Civil Law system).
I think you are focusing on legislative or statute law but I will briefly cover the others first.
Judicial or judge made law occurs when a court makes a judgement on a case about what the law means. In a common law jurisdiction such decisions are binding on lower courts in the same jurisdictional hierarchy and persuasive on the same level and higher courts and foreign courts. In the US foreign courts normally means courts in other US states that lack a binding precedent on the matter but courts can look to decisions in other common law countries like Canada, Australia, the UK etc.
Because courts interpret the "common law" their judgments usually reflect (in some way) community expectations. However, judicial systems are, by their nature, slow to change and while they track community expectations they do so with a time lag, say 20-30 years: the lag in time between when the judges making decisions now had their views on society fixed during their college years.
Judicial law, in theory, stretches back to Anglo-Norman law with decisions building on (and occasionally knocking over) what has come before. In practice, this chain has usually been severed at some point (usually more than once) by the intervention of the legislature to correct what they saw as a "failure" in the common law. Subsequent common law builds on that statute like barnacles on a ship's hull (but without the negative connotation ... maybe).
By your terminology, such laws are almost always descriptive.
Is the law created by the administrative arm of government using the powers explicitly granted by the relevant constitution, delegated by the legislature or derived from common law powers. For example, if a statute authorizes the levying of taxes on the value of real estate, it is usually left to the administrative branch to determine how to collect and use the data needed to arrive at a given property's value.
By your terminology, such laws can go either way. Establishing the land tax system example would be prescriptive, modifying it to match changing social circumstances would be descriptive.
Legislative or statute law is the laws passed by the legislature by whatever name that is known. They can only make laws within the bounds permitted by their constitutions but within those limits they are free to do whatever they choose.
Many of the laws they pass are simply to enable the other parts of government to get on with their jobs - like giving them the money to pay the army or the judges or whatever. I guess these would be descriptive - of course, passing a law to say you can't pay the army would be very, very prescriptive.
Aside from that, while legislatures can pass a law about anything they generally only intervene when there is some "mischief" in society that the current laws are not addressing or are addressing in an unjust way. This happens all the time because society does not stand still: we didn't need laws about cell phone use in cars until there were cell phones being implicated in accidents. Similarly, we didn't need laws about Pokemon Go until a week last Thursday (more or less). To that extent their laws are prescriptive.
