Earlier quoted context omitted.
I guess I worded that poorly; it isn't merely that we don't need an objective measure: we literally don't need a measure at all, as the crime would be attempting to cause it, whether or not it was even possible to do, and so we simply do not care if the activity was addictive. If you are going out of your way to exploit the psychology or physiology of other humans in an attempt to use that to sell your product, maybe…
This just shifts the question to: what counts as attempt to cause an addiction? Does writing a book aims to make people read addictive? Try to design a gym class that makes you feel good about yourself so you will come again? You still need to define what is addictive
We thereby do not necessarily need a way to know whether reading--either your book or any book--is addictive or not, but only the extent to which you were going out of your way to make it addictive, for which I think it might then be OK to have some specific-yet-contrived definition that is difficult to apply to any specific product but feels like a wrong thing to do (maybe my "exploit human psychology or physiology")?