Why is there such a big thing about prescription opioids when the addiction rate in less than 1% for those who have been prescribed for pain?

How exactly do they count addictions when someone has an ongoing prescription?

Because it's low hanging fruit. Politicians can be "tough" on chronic pain patients and frame it as furthering the battle against the "opioid epidemic" without having to address the actual kraken in the bathtub, which is shady, Chinese bootleg fentanyl flooding the US through our leaky International mail system. The opioid crisis and epidemic of overdoses isn't fueled by Grandma selling off her "extra" 5mg percosets, its coming from illegally smuggled fentanyl being sold illegally on the streets. Chronic pain patients are just a convenient sock puppet for the constant campaign mode our politicians are in, and they don't care who they hurt to get elected or stay in power. Imho.

Your 1% figure comes from misrepresentation of a 'study', pushed by Purdue and others for criminal gain.

The One-Paragraph Letter From 1980 That Fueled the Opioid Crisis

Purdue Pharma, which makes OxyContin, starting using the letter’s data to say that less than one percent of patients treated with opioids became addicted. Pain specialists routinely cited it in their lectures. Porter and Jick’s letter is not the only study whose findings on opioid addiction became taken out of context, but it was one of the most prominent. Jick recently told the AP, “I’m essentially mortified that that letter to the editor was used as an excuse to do what these drug companies did.”

Don't get me wrong, pain is miserable and treatment needs to be better. But around 80% of opioid addictions start with prescriptions for people in genuine pain. What percentage of prescriptees that is, I don't know. But it's not a trivial issue, and it is a very difficult problem to solve.