Ok, the plot thickens. Upon further research, I found the attached warning/note on the Charlotte website.
So this opens up more questions. Why male threads only? Seems to me the male fittings go inside of female fittings, presumably metal or CPVC, which would prevent the fitting from expanding beyond its elastic limit and prevent failure. And, more importantly, why is the NIBCO fitting NOT subject to the same limitation? Does Charlotte make inferior products? Doesn't seem likely.
The other baffling fact is that Charlotte makes that general statement for all sizes of fittings (diameters). Clearly the pressure rating of pipe and fittings go down with increasing diameter so a smaller fitting, say 1/2", should be able to take a lot more pressure than a 1" fitting.
Any engineers here with a plausible explanation?