It is one of those "one person's X is another person's Y" comparisons: indoctrination/brain-washing, religion/cult, freedom-fighter/terrorist, cute cat / insane evil sharp edged ball of allergens, ...
A more passive form of indoctrination would be repeated explanation (with or without query/prompt) so explanation can become indoctrination depending on context, but I'll stand by my choice of words above.
"Where do rainbows come from" can be answered with the explanation "As a way to show God's promise to Noah never to flood the Earth again" or with the explanation "because sunlight refracts through water into all the different colors we can see" or with the explanation "invisible unicorns fly through the sky sometimes and leave rainbows where they've gone".
Many people would call one or more of these indoctrinations, despite the complete lack of "should" in any of the responses. Egocentrism does not a benignity make.
Besides, just as you hold your "must be provable" views dear, similarly the religious parent holds his or her religious views dear.
Adults just slow down the writting but we always try to fit in the social group that we are living. And that includes political opinions, faith, etc...