Dead Ball , Incidental Contact ?

[Chess Ref]

I have read the threads concerning dead ball fouls need to be Int, flagrant or lignored. Until it happened in my game it was more a theory than something that could occur. It occurred.

Player A makes his free-throw. As they start going the other way Player B gives a forearm push to Player A2. The ball was not at the disposal of Team B yet. I tweet . I report a foul, no one says anything. Games goes on. I am pretty sure I got this one wrong.

1. Am I correct in that this foul falls under the Int, flagrant or let it go Sitch ?

2. Assuming this foul falls under the INt, Flag, or let it go thingie I really don't like that there is no middle ground. If this occurred in a live ball Sitch i would call a foul but not an INT. So any thoughts on why it has to be called an INt or ignored ? I am looking for reasons behing this rule.