POSIX specifies that each (short) option name "should be a single alphanumeric character (the alnum character classification) from the portable
character set".
In order to allow implementations of SRFI 37 to use the POSIX/GNU C library functions and to make the defined command-line interface interoperable, I think it makes a lot of sense to add this restriction to SRFI 37 as well.
This is even more important today than when SRFI 37 was written because most Schemes nowadays understand Unicode and thus the character concept of Scheme is no longer the single character, short option concept of the POSIX interface.
Marc