In a comment on this question that is now deleted, DoubleAA said something to the effect of
Yes you become impure by sitting on the seat, but anyone who needs to care about the consequences of becoming impure knows about them and acts accordingly.
(someone feel free to edit if you have access to deleted comments)
What are the consequences nowadays of becoming tamei from midras? We don't eat terumah or kodshim or go into the Azarah because we're all assumed to be tamei from dead people anyway.