Well why is it that it's always Dortmund that seem to get caught out with them? I can't recall many top players at any other clubs that have these clauses in their contracts. At least in the case of Reus they could have insisted on a higher release clause. £30 million would have been very reasonable. His release clause is laughably low no matter what way you want to dress it up.
Because Dortmund worked their way to the top very quickly/recently and you probably compare them to clubs that have been among the European elite for years/decades.