[Yamer]

Well, they have established that people can get very ill from spreading the dead on them (Gabriel's infection). Why they waited 8 seasons to do it is a valid question.

Flinging the dead into the Hilltop is good old fashioned siege warfare. Unless somehow they wound the people they are going to have start making up some more new rules for this kind of strategy to be effective.

We haven't seen what happens to people with tainted food/water, though it has been implied that it's not something that you want to test out (the bloated well walker, slaughtering the pigs at the prison, the Terminus crew eating an infected Bob leg, etc).