this is indeed a misfeature, and infects all the ams document classes.
it was introduced owing to a misunderstanding in interpreting the specs provided,
and is on the list for reconsideration when the ams classes are updated
(but this is not scheduled).
the (re)setting of the indentations is in the block of code beginning at line 709
(this is preceded directly by the line
the offending instruction is on line 714:
\advance\leftmargini by \normalparindent
if that line is omitted from the resettings, the indentations will be much
better behaved. however, i haven't managed to come up with a simple patch,
so the best advice i can offer is to copy the entire
into your preamble, wrap it in
\makeatletter ... \makeatother, and exterminate
the line that includes
in response to a comment, the misunderstanding when writing the code was due to the "usual" bureaucratic foulup.
as to why it is conceptually wrong, there are several reasons, all stated here
(as my own opinion) with respect to ams publications:
the extra indentation takes up too much space, and for long lists, pages
look unbalanced for no underlying stylistic reason. (a justifiable
stylistic reason might be to provide space for side-aligned illustrations.)
especially for long lists, more pages are required, unnecessarily, leading
to higher production costs and higher selling prices.
for "small format" books (the ams publishes several series with a 6-by-9 inch
trim), the space allotted to text is too narrow to accomplish attractive line
breaking, especially with math, in turn making this material hard to read.
ams publications typically contain a lot of display math. within lists, then,
the horizontally centered displays look particularly off center, and centering
them within the width of the list text (a request seen not infrequently)
leaves too little room for effective presentation of display material.
while it is sometimes said that what output looks like doesn't matter, as long as
the content is correct, there are those who do care, and effective use of space
to enhance the readability of content can be an important factor in the perceived
quality of the material.