The reason they allow this has actually already happened.
Think back to the days of early DP, the GTS was littered with level 9 and under Dialga and Palkia. Now the simple, not-thougt out answer would be 'Ban any that are under "Level 50 and Under"!'. but see, Game Freak kept the option open, because, low and behold, HGSS had an event that would net you a Lv 1 Dialga, Palkia, or Giratina.
Outright Banning these so-called impossible trades, would make it so people who want to offer up their event can not do so.