The goal of this kind of challenge is to use active spells/items as effective as possible. You should rather outsmart than outcreep the opponents. Passive creeping weapons or spells like the tinker towers are exactly what does not fit into this way of playing.
(One could argue that based on this reasoning tank cannon should be banned as well, but I don't think that it is necessary.)
(One could argue that based on this reasoning tank cannon should be banned as well, but I don't think that it is necessary.)