This is basically the "security through obscurity" argument that has been so heavily criticised and largely shown to be false. If they (and apple etc.) Open sourced their approval algorithm and allowed pull requests people with an interest in having a high quality app store could try and poke holes in it and make it more robust. I know there are humans in the mix making judgements, but those judgements are largely of the "does the app meet this tick box criteria" type.