The penalty is there to prevent bouncing the voting power back and forth. Otherwise If i lend you my steempower and you vote, then i'd take it back and i'd have the power at my use again. That would allow manipulation by pumping up posts to create attention, and then redoing it on other post with another loan group. Of course you would lose the curating rewards from that post, but possibly gain author rewards from another. Would it be beneficial, i can't tell.
So problem if i would set tax 100 percent i would get 100% of your rewards. Then it would be close to actual transferring of steempower from rewards. Say i lend you 1000 steempower and you have 10000 of your own. And if i had more and taxed you less, so you could possibly get bigger proportion of steempower as you would normally. By keeping the rewards based on actual share you would not have these irregularities. But would it hurt if they were split differently, again don't know.
Ok the first point is definitively good, a penalty should exist. About the tax 100 percent that's up to the user to decide will he accept or not, obviously I wouldn't necessarily accept someone lending me 50.000 SP but taxing me 100% for it, that would mean all curation rewards would go to that person. The ability to set custom "tax" is there in order to be flexible, someone would like to power up a friend by taxing him less so that the SP gain of the friend would be increased.
That is "desired", that can be used to power up good curators.