Well, why not?

This gets into what exactly "speeding" is. As a simple definitional matter, to "speed" is to drive faster than whatever the speed limit is; more finely, it is to drive faster than the government says you're allowed to. Okay. So what? The government also says we're not allowed to do a lot of other things. Does this justify punishing people when they do them?

Why?

The government's response is that "speed kills." Well, if it does, then we're all dead – because the Earth is "speeding" along at thousands of miles an hour. Of course, the government will say that driving any faster than the speed limit in a vehicle is what kills. And yet, people get killed every day driving at speeds that are below the speed limit – and it is also true that millions of people don't get killed even though they drive faster than the speed limit.

So, clearly, "speed" does not "kill."

At least not necessarily. Thus we tread on questionable ground, which is the natural turf for abuse. About 112,000 speeding tickets are handed out every day in the United States. Forty million annually. These tickets generate on the order of $3.8-5.4 billion in "revenue" for the cities, counties and states that sic cops on people for "speeding." 

You might call that a profit motive.

Everyone (including cops) knows it's basically a vast, systematized mulcting operation. Yes – of course – there are reckless drivers and "speed" is often a factor. But the majority of "speeding" tickets are handed out to drivers who had the bad luck to be the ones that got picked out from the crowd that day. Everyone was "speeding," but the cop chose them for them the ticket – because someone's got to pay.

They are like the fish that swim in aquariums at some restaurants where you get to pick the one you're going to eat. It's your day to get picked sometimes. When it happens, both you and the cop perform the absurd ritual of pretending the interaction is legitimate. The driver apologizes for "speeding" – conceding the stop is legitimate when of course he is well-aware he's just being mulcted, hoping for a "break" – while the cop pretends to be the enforcer of "safety," never mind that he almost certainly drives faster than speed limit himself all the time. It's of a piece with being friendly toward the property tax assessor,, whose guts you hate – but you don't let him know because he has power over you (and your wallet).