If the Idon'tminders felt certain that they could express their preference, be heard in an equal way, then they would speak out! If the Idon'tminders in your life cannot risk giving their preference, then why don't they feel safe to do that!?
There you go again with the high-stakes vocabulary of 'risk', as though saying 'Actually, I'd like to eat Thai' is like climbing into the deathzone on Everest. What exactly is it that you think you are 'risking' when you say this?
I am not denying your specific experience, obviously, but I can certainly tell you from a longtime study of a chronic 'I don't mind'-er in my own life that in her case, it is a refusal to take responsibility for absolutely any decision, however small, because the consequences are then on her. It's much easier for her to say she doesn't mind, leave the decisions up to other people, and then present everything as Not My Choice So Don't Blame Me, even when people around her are begging her to decide because it's something she's more informed about than they are.
I recently went for a walk with her in a place she knows well and I don't know at all. There were two possible routes, of different lengths and challengingness, and she has some health issues, so as she knew the terrain and I didn't, and only she knew what she felt up to that day I said she should decide which way we went. But 'I don't mind' is all I got.