I think the answer is going to be, it depends.
For a lot of people in the US, they will have to drive to where they are planning on walking. For those cases, having jeans on and getting wet wouldn't be as much of a problem.
There are also a lot of places in the US that are warmer, colder, and/or have less rainfall than the UK and Europe do, so for those cases, as long as the person is comfortable in jeans, then it shouldn't be a problem. And if it was cold, I would definitely be more likely to pick jeans over something lighter.
There are also places that naturally have a lot of brambles, thorns, and sticker bushes, so for those cases I would likely wear jeans rather than something lighter, just to prevent skin damage. Not to mention barbed wire fences, if you are going across someone's property (with permission).
I think part of it may be that, for a lot of outdoor uses, jeans tend to be more durable and protective of one's legs than something lighter, so that makes it the default option. For bad weather, if you aren't going to be in the rain for long, it isn't too bad, and if you have a long enough coat/jacket, you can keep your backside dry even though your legs get wet.
Of course, if the person is hiking for days, or will be hiking in rainy areas or hot areas (Death Valley, desert in general), I would recommend wearing something other than jeans. Jeans can chafe after wearing for a few days, and they are much heavier than most other options.
What do you wear for hiking/walking?