bug#47085: du: why does 'usage' show prefixes 'Z' or 'Y' if they are disallowed?

[L A Walsh]

I thought to display 0 (or 0<magnitude>) for 1st arg by doing:

du -BY, as -B says I can list a unit for scaling, but for
-BY and -BZ I get:
du: -B argument 'Y' too large.

It doesn't even look to see how much space is used, it
immediately returns Y & Z are "too large".

Why are those suffixes listed as valid under the program 'usage'
and manpage, when they are automatically disallowed?

Also something I didn't see as being clear:
 I don't see where it says it will always round up to next higher
 unit (it may say it somewhere, I just don't see it and was
 surprised to see 'du -BT /tmp|hsort -s' show all objects as
 being 1.0T and for hsort to show a summary of ~148T
 (hsort sorts by human prefixes and has an optional '-s'
 switch that shows an additive sum of the sizes).