Ah! I thought we were talking about the Imperium of the Great Crusade in the Horus Heresy series, not the contemporary version. My mistake.
Just to run with that point though, I was thinking more along the lines of how the conversion to Imperial Truth during the Crusade took place and how the populations were after this process had been normalised, rather than suggesting that there was much freedom to choose ideology after Imperial conquest.
So, for instance, the conversion of western Europe to Christianity wasn't by the sword per se
, it was by constantly exposing the indigenous cultures to the new ideology at a grass roots level. The persecutions of non-Christians came much, much later after monarchs started to declare themselves divinely mandated and so requiring the enforcement of the 'new' religion and ideology on non believers. The monarch's authority and the new religion became one and the same. I'm not saying that the Imperium only preached changes during the Crusade, they obviously also enforced them very harshly in many contexts. My point was that the approach was multi-tiered, and even then, after conquest and normalisation of the new ideology, the question we need to ask is how the population saw themselves. Did they feel in control of their own destinies within the context of the Imperium? Or, at least, did they feel that they had been treated honourably by their enemies?
Obviously in the cases on Angron and Curze the answers to these questions are probably 'no', but I think the image we're given of the Great Crusade is that the Imperium of the Great Crusade era went to great lengths to justify itself through both word and deed. So by being consistent with their (non-tainted human) enemies and treating them honourably, so that when these societies were finally conquered they didn't just feel that they had been bulldozed by an evil oppressor who treats them like scum. The idea was a little bit more like the older image of the Tau Greater Good - "we conquer you in order to bring everyone together into the same grand project of collective security and 'enlightenment' - you may not see it yet, but judge us by our works after bringing you in to the Imperial fold, not just by the fact we warred against you in the first place".