Don't know about other states but I don't think that is true in AZ. That said, I often drive less than 25mph on residential streets. The reason being parked cars and playing children. Some of the minor arteries make it unsafe to drive over 25 for the same reason.

As many LEOs have become accustomed to and count on.

I've never heard of that one before.

Other than the Interstates or some divided highways, I haven't seen minimum speeds posted. I checked our state law, there's a mention that a posted speed limit must be 15mph or more (a minimum for the maximum). Some roads do not have a posted limit.
I know we have roads with 15mph limits, particularly around school zones. My neighborhood is 20mph. So the 25mph minimum is apparently not valid here.

And every driving manual I've read has always emphasized that it's the operator's responsibilty to limit speed according to conditions, no minimum speed is set.

I believe there is a misinterpretation of the law here. I bet the law says something to the effect that communities are not permitted to set speed limits (i.e. maximum speeds) less than 25 mph. Requiring drivers to drive at least 25 mph is rather silly: "Son, do you know that you were driving 7 mph under? What's that? You were concerned about the ice on the road? License and registration, please."

Around here, they install speed humps on 25 mph roads that force cars to slow down to 10 mph (and get in my way!)

Sooo ... 7 pages here and still nobody has a friggin clue why the LEO chastized this VC road HOG.
Well it's because he couldn't even share the 12 feet lane with a Motorcycle. This BS about excluding anything and everything from the entire lane width is absolute total nonsense. Ever see Hell's Angels ride single file ??? hahaha
Riding in the left track of the left turn lane is rather goofy also.

As for minimum speed limits for normaly capable vehicles, I am sure for highways it is generally 10 mph below the max.

Yay, we have an expert!

That is a separate issue from driving according to conditions. Countless times, I've been in conditions, while driving a "normaly [sic] capable vehicle", where 10mph under the limit would be courting suicide [meaning 10mph under would be on the fast side of safe, i.e., in blizzard, icy, heavy rain, and heavy fog conditions, for example].

He's not required to share the lane with the motorcycle, any more than than a motorcycle is required to share the lane with another motorcycle. It's permitted, not required. He's riding the sharrows, exactly where he's supposed to be. What you're saying is he should be obligated to put himself in the door zone even though road markings clearly indicate otherwise.

This is completely off-topic, but countless communities set speed limits that are lower then the state limits. New York City, as example, has a typical maximum of 25 mph on local streets. Which pretty much means they are not giving tickets out for going too slow. And of course, you get designated zones where the speed limit may be as low as 15 or 20, school zones being typical.

I have seen minimums, but mostly on "freeway's", interstate highways and the like, where I have seen 45 as typical as a minimum. This is mostly to avoid somebody driving up the ass of a slow-poke while they're going 20+ over the limit.