Well to be fair... rivers flow towards the equator... So us "Other Hemisphere" people are used to north-flowing rivers.
It's about "in general" rather than "all and every". In General, rivers flow to the equator. In General, lakes have single outflows and unify (rather than divide). In General, rivers avoid higher-levels-above-sea-level.
So, yeah they'll tell you the rivers should be heading south, because in general that's where they should go. The Nile probably has fair reasons for being the exception (I'd assume there are mountainous or higher-level-above-sea lands to the south)... but it's not just because "I want one that goes north". Unless you have really quite specific reasons for 'not following the general rule'.