It's just a python script, works fine here. Maybe there should be an inherit for "small scripts" that'd set a reasonable base KEYWORDS?
The reason an eclass is not suitable is because scripts are not necessarily architecture-agnostic. It's rare, but it happens. They are often not environment agnostic, and as Gentoo is ported to Mac OS X, the BSDs and others, things like this will occasioanlly break. It is widely held that the list of packages that can have all keywords added at once should be as small as possible. Marked in CVS, thanks for the report.
*** Bug 74099 has been marked as a duplicate of this bug. ***