PDA isn't recognised everywhere yet. It's a specific branch of autism. Autism already has demand avoidant behaviour, so be diagnosed PDA needs it to be really impacting.
We aren't quick to label children, the diagnostic process for autism takes many years. Took my son 3-4 years. He is PDA, and the diagnostic team did not want to include it, but one of his advocates fought to get it put on his diagnosis. Because PDA autistics need vastly different techniques to support them.
It is absolutely not used as an excuse for bad behaviour, and I and his school work tirelessly to support him to become an adult who can deal with the PDA behaviours.
PDA autistics live in a constant state of fight or flight. Imagine how exhausting that must be. Everyday demands, such as toileting can be difficult for people with PDA. But they aren't unteachable, if fact with the right atmosphere my child has gone from being incredibly violent on a regular basis, to very chill. We rarely have meltdowns anymore, because he's being helped to learn how to cope with them.
He is excelling in school and has been nominated for an award for his progress at school. When just 8 years ago, I didnt know if he would be able to access any schooling at all.
TLDR: PDA is a very useful diagnosis because of the difference in support needs from other autistics. And with the right support, they can and do excel at whatever they want to do!