Jannie asks "Why are scrum halves allowed to put the ball in towards their scrum, under the hookers feet?"
Well Jannie, normally when people ask this question they say "why are scrum halves allowed to feed the ball into the second row!".
The simple answer is they aren't allowed to do that. The law states:
"The scrum half must throw in the ball straight along the middle line, so that it first touches the ground immediately beyond the width of the nearer prop’s shoulders."
So what The Rugby Ref thinks you are really asking is "why don't referees penalise the Not Straight?"
Most referees at grass roots level will penalise the "not straight". Sometimes though the referee is distracted by other more important safety issues, such as binding, dipping and lifting.
Why don't they penalise it on the television? The Rugby Ref doesn't have an answer to that one, but has been told in the past by top referees that "there are many things to look at in the scrum and the feed is one that is a way down the list, below important safety considerations".
That is not really a satisfactory answer, as ignoring the feed means that any prospect of a contest for the ball at that point is removed. Leaving just a pushing contest. The Rugby Ref has noticed of late though that more top level referees are now policing the put in more often. Maybe the tide is turning back?