if you want to use logic, you should ask about HOW exactly demigods do destroy towers, and what is there problem while doing this. and towers on high has nearly no impact on most of the demigods and a far to less to some few of them that it does not impact balance in an important to note way.
to explain this a bit better:
1. for all dgs it has the consequence that they need more time to destroy it. this is balanced along all dgs.
2. some dgs can do this without taking dmg, or wihtout taking significant more dmg then they regenerate (reg, tb, sedna, partially, qot, all minion build dgs, spit ub)
3. there are
of course some demigods that cannot avoid a significant amount of damage when attacking twoers very early in the game - this are ooze ub, assasin oak, assasin ereb and rook wihtout this tower eating skill.or without towers)
now for the examples in 3. they are either rare or they even do not bother much for early tower destructing anyway, no matter on what towers are set. hence it total it does not have a significant impact on gameplay and balance if you set towers to high except that it may take somewhat longer, thus the level the first towers falls is 1 or 2 levels later then on default setting.
but it does not impacted much the balance between demigods.