I completly agree with your point, that when someone asks a question, them replying
"just asking" as the reason for asking the question would be completly false.
Whenever an individual asks a question their reason behind asking it can always be categorised as either curiousity or purpose.
For example, say someone was going camping and figured he would need to light a campfire to make the trip enjoyable for others. For this reason, he would ask how to start a campfire, and if he replied
"just asking" to prevent explaining his situation, he would be saying a false statement as he had a purpose for asking his question.
By contrast, if someone wondered what was needed to start a campfire, him asking that question would be due to curiousity. Thus if he replied,
"just asking" he would be saying a false statement as he is being driven by curiousity, and in all likely hood this curiousity would have been derived from something else (watching it happend in a movie, reading it in a book, talking to some friends or thinking about a situation which involved a campfire). Thus he would have a reason for asking his question curiousity based of a specific thought.
The only reasonable situation where,
"just asking" may be a true response would be if someone decided to conjure random statements (purely random), firstly, the question he or she would genereate would have to be based of some curiousity, (even the smallest), or would have to be plain-expressionless questions like
"Whats 152+76?" or
"How many letters in fickle?". However, if he was asking these questions with absoloutely NO interest in the answer or for no other reason, his purpose would be to either, justify a reason to respond with
"just asking" which to the contray would give him a purpose other than
"just asking" or his purpose would be to simply ask questions of no interest to himself (the activity he is doing!) hence he would have a purpose!
Thus the responsive statement "just asking" is ALWAYS false.
If you disagree, I disagree (with you), so feel free to try firing some cases at me.