Basically, certain big buyers/sellers of stock want to be the ones to benefit from the information "Someone really wants into or out of this stock", and they really, really dislike when the HFTs democratize that information advantage faster than the larger players can gain extra returns from using it.

Trading communicates information to the market. Proprietary access to information is valuable. HFTs make certain information non-proprietary faster than the state of the art prior to HFTs. That changes the allocation of profits associated with that information. This is the entire ballgame. Retail traders swim placidly on the sea above totally ignorant of the krakens vs. mutant-sharks-with-lasers bloodbath happening 20000 leagues below.