There are situations where allied preselects warrant a ban. When a new champion is out, banning them regardless makes sense cause there's no way anybody is good at them yet. New champs are wildcards; nobody has true skill with them for a while. There's also the problem of clearly OP champs. If your last pick wants an OP champ, it still makes sense to ban him as the enemy team is likely to get it first.
However, there should still be a penalty that makes it to where you have to seriously consider whether or not to ban an ally's preselect. Personally, I think you should lose double the LP from a ranked loss if you ban an ally's preselect. Though now that I think about it, making it to where you can't ban an ally's preselect in normal draft does make sense since that's where you're supposed to practice with and against new/OP champs.