I'd say this is complete garbage. At this point, anyone claiming that there is a big difference in literary influences in a modern genre like this is cracked.
The assertion simply isn't held up by the wide variety of product out there. Take something like the Hannibal/Silence of the Lambs series of books. While originally focused on a single case, "plot driven" if you will, this series came to center around one character, the at-the-time unique Hannibal Lechter. Although Hannibal Lechter has become a stereotype since then, at the time he was unique.
Let me continue. I'd even dispute that the average crime novel can be non-stereotypical, as 90% of them are just yet another variation on Sherlock Holmes. A quirky detective and a stolid partner. For that matter, Sherlock Holmes stories are mostly plot driven, as much as people love the little details of Holmes, who can deny that these stories focus on a single case above all? Doyle alone disproves your theory.
I'd bet the person that advances this claim first makes a stereotypical claim about what "American" or "British" fiction, selecting examples to support their theory. I find it doubtful that they actually look at the wide selection that is published in this field yearly, but narrowed it down to favorites or hatreds. They most likely proclaimed Agatha Christie represents all British novels and that Hammett is all American novels.
As for societal influences, sure they make minor variations. Class is obviously different. For that matter, no American writer will have plot that revolves around rail way time tables, while that has occured enough in English Lit for it to have been parodied. All such things ae merely cosmetic.
