That's not totally accurate to say that's the biggest concern. The main risk (by a long way) with an apple is disease or pests that may be arriving in or on it.

If that apple comes from a tree that was infected with fireblight, and you go to an apple growing area and chuck the core out on the ground, you could have potentially decimated our apple industry by spreading a disease we don't have here. All of a sudden China (for eg) won't buy our apples because of the risk, and our farmers lose millions. Then farmers go broke trying to reactively manage the sick trees and try to stop it spreading...

This also applies to live plant imports.

With unidentified seeds of course there is a potential weed risk, but about 80% or more of the time if your seeds are labelled then they will be checked for any signs of pest or disease and then allowed in. This does NOT apply to seeds for our important commercial crops, btw. Like corn, rice, wheat, tomato, capsicum, pumpkin...where there are viruses that can be passed on inside the seeds or the industry is too important to risk (eg cotton).

Plus, it's biosecurity (more commonly known as quarantine) that checks for pest and disease, not customs. Customs deals with drugs, firearms, alcohol ans associated tariffs.