The pool operator knows or has cause to know it's a flawed system and by continuing to offer it accepts that hopping will occur.
The miner knows or has cause to know that proportional is flawed and will be hopped, thus cutting in to their earnings.
Neither operator nor miner are forced into their actions and alternate (many!) exist. Ergo, it is not unethical.
Just because something is possible doesn't mean it is ethical. The scams going on in the loan thread are possible
because there are some "suckers" ( including me) that fell prey to the scammers. So is it ethical for them to do it
just because they can? When I started mining i had no idea about hopping and of course was on prop. pools because that was mostly what there was. When it became more public knowledge I was pissed and was against the idea, and of course moved to other pools and of course spoke out against the hopping system. I remember ( i think...) that organofcorti said once that he was doing it for the good of the miners - his rational was that it will eventually push all the prop. pools out of the market...meanwhile he made some nice btcs....
EDIT: forgot to say that I am mining on this pool because I accept the OPs explanation for now.