That's not the kind of restriction that stops pool mining. The trick is to enable the pool members to steal the blocks they discover. Andrew Miller, a grad student at UMD, has an ingenious scheme for doing this. I am pretty sure I put the link in the article, under the first bullet in the "What to Do Now" section.
Wouldn't pool participants that use this extension to steal rewards be exposed to the pool simply due to their work being consistently challenged and thus lost?
i.e: The pool would notice that certain participants contributions are conflicting with other discoveries, and ban such participants?
Fees cost almost nothing to hostile miners. With a 20% fee, they get 100% of their hashrate through the theft, and 80% from the pool. So long as you pay anything at all to new miners doing this attack is beneficial.
