Hans Aberg asked:

>   I find this interesting: Do they state a reason for this recommendation,
> so that there is a general principle behind it, like the idea with
> typesetting constant names upright?

You may call it general principle. The following conventions are often
employed by physicists:

Variables:         math italics
Vectors:           bold math italics
Operators:         upright
Vector Operators:  bold upright
Tensors:           sans serif

Constants are usually also in math italics (generally for physical
constants like speed of light $c$), only \emph{numbers} like e, i, and pi
occur upright depending on the publishers style. Differentials are also
often upright.

If you want to see a journal which follows all those conventions, look at
Il nouvo cimento.

--J"org Knappen