When you see sandals on an American man, thank a beatnik.
In the 1950s, it would have been nearly unthinkable for a man to dress that comfortably.
Really, we were that uptight.
Now?
They’re everywhere.
At least in summer.
Remember, where I live, we get snow – lots of it, some years.
Can’t blame a beatnik for that.
Jnana's Red Barn 
Here in Aotearoa New Zealand, sandals, or more specifically jandals (“thongs” to Australians and “flip-flops” to almost everyone else) have been standard casual footwear (and not so casual) for decades – certainly since the late 1950s. In the 70s and 80s, Roman sandals were acceptable as business footwear for men when worn with socks that came to mid calf or higher and dress shorts. Less so now.
Here casual footwear is just as likely to be bare feet. You’ll see that anytime you visit a supermarket, although admittedly, less often during winter. We are definitely not uptight, but Americans have a very long way to go to be as laid back as we are 🙂
Wonderful and welcome perspectives.