Why does the council of vocations assign Equality to the job of street sweeper ? Is this due to error or incompetence, or more sinister motivation ?

We aren't told exactly why the Council assigned Equality to the "Street-sweepers". We can infer that his teachers found him too curious... too intelligent, and it wasn't about intelligence..... it was about keeping everyone ignorant. As they say, ignorance is bliss, and ignorance keeps people from asking questions.