It's not that medics don't know how to differentiate who needs to go, it's just that most of us do not have a mechanism to send patients to other places.
That is not supported by the evidence.
http://www.ncbi.nlm.nih.gov/pubmed/19731166
http://www.ncbi.nlm.nih.gov/pubmed/12385603
http://www.ncbi.nlm.nih.gov/pubmed/14582100
Infact, every study on the topic shows paramedics cannot effectively determine who needs to go to the hospital. Though sometimes they can sometimes determine who needs transported to the outdated definition of what an Emergency Department is used for.
The reason is very understandable. Supporting what USALSFYRE said, paramedics and EMS in general do not have the education for it.
A patient who does not need emergency resuscitation may still need to be admitted to the hospital. For reasons ranging from observation to more aggresive treatments for common conditions.
An easy way to measure is for you to keep a log of all patients you think don't need ALS or transported and compare that to the ED census, including non-ambulance transports, who are admitted or treated in the ED with interventions not readily used at home or require advanced diagnostics. (like simple suturing)
But I think you will be shocked by what you find.
Keep in mind, EMS education is not general medical education. There is a lot more in medicine than hemodynamically stable and unstable patients.
Because of the dysfunction of the US healthcare system, the ED is the primary entry point to it now. To deny a person transport without alternative destination or treat and release is to deny them access to the healthcare system.
After much discussion with US EM physicians, even they claim their primiary role and value is in determining who can be safely discharged without being admitted.
That would mean paramedics are trying to replicate that ability with OTJ training and no follow-up.
If we could leave patients home without the agency screaming "liability" or call a cab to transport the "not feeling well" patient to the urgent care, I'm sure we would. I know I hate bringing most of my patients to an already busy, over crowded ED.
This would be a very good use of the EMS system, to act as basically a healthcare traffic cop.
Unfortunately the education is simply not supportive of this role. Also don't forget, this will be taking money away from Emergency Physicians, and they will offer any and every excuse why this is a bad idea or cannot be done in order to protect thier money.