Dmitriy, yes, and that's the point - in practice Wikipedia categories are not transitive.
We've been working on this slowly at pl.wp lately, trying to split the categories into two kinds marked with appropriate templates: "topic" categories, including all related entries, e.g. Category:Water; and "object" ones, which would imply transitiveness, like the Politicians example provided above.
Topic cats could contain object ones, but not vice versa. This seems like a good compromise to me, but requires a good deal of work, and we're slowly progressing.