Not bad, but...
8. Give it a rest. Turning off your computer when it's not in use saves energy and clears out the RAM, or temporary memory, which would otherwise slow your machine over time.
I think this highly depends on the OS you're running. On Windows, sure, I'd agree with this, but I've kept OS X and Linux systems up for months on end without even a hint of a performance decrease.
According to Microsoft, Windows XP requires 2 weeks downtime per year...
However, we have computers at work that see no downtime whatsoever (the place I work at is manned 24/7, so it's just one person logging out and another logging in) and they work like a charm (Win XP). Other haven't seen a reboot for years, but those are a special case, because there's only one or two apps running on each constantly (things like antenna controller software).
The oldest ones, running DOS, have been running 24/7 for the past 15 years.
Is this commoner computer stupidity only rampant in the US or has it infected other nations as well?
No, they're everywhere.
I was under the impression that the constant change between power states (on and off) was as hard or harder on computers than just leaving them on. Am I wrong on this?
Nope, you're completely right. However, the changes in lifespan are negligible, so it's safe to assume that whether you leave it on all time, or shutdown frequently, the lifespan will be more-or-less the same.
I don't blame people for being ignorant though. Spend over $100 on Windows and look what you get as the instruction manual.
There's Help, which covers a lot of stuff. Apart from installation instructions and basic troubleshhoting, an electronic Help is a better solution, because it has a nice function called "search"

I got a 450-page manual with my Nikon D300 - you have no idea what a PITA it is to read such a thing, even though Nikon did a decent work of keeping stuff organized. Thank God DSLRs are made to be intuitive, and that manufacturers don't make revolutionary changes between models, makes it much easier to sort out stuff yourself.