I don't think due process is a waste of public money. The officers were of the opinion an offense was committed and the everybody has the right to put their side of the story.
The judge hears the evidence from both and then rules, the officers thought they had enough to prosecute and the judge's opinion was it wasn't quite enough. That's due process, not waste.
and as for being on the phone isn't a big deal, I was watching police, camera, nonsense on sky and a woman, on the phone, pulled out the wrong way of a garage into incoming traffic and wiped out an oncoming car seriously and permanently injuring it's driver that seems like a big deal.
I wouldn't be suprised if the coppers did follow you home, but if you are driving legally that's not a problem is it.