I think the standing is about not showing disrespect, rather than showing allegiance.
However I do very much take your point.
The instructor proceeded to explain that, instead, it is announced that John Doe has disassociated himself from the congregation.
Exactly FFreek 2005 it's a distinction without a difference. In this case they don't DF purely for legal reasons. Same for taking a blood transfusion. If watchtower kicked people out for joining up, taking blood or voting they'd be prosecuted by governments because every citizen has a right to those things in a democracy. Suffrage is universal and they'd be in hot water pretty quick if they forbade that, too. So as with joining up they DA.